My husband would use the phrase "want in one hand, .... in the other and see which one fills up first" about wanting another child. You are certainly under no obligation to support them especially in lieu of her wanting another child when she already has two that they haven't been able to support without your help. I have to agree with you that this is very irresponsible given their circumstances.
I strongly support one parent staying at home with their children for the first couple of years if at all possible. We are currently subsidizing our son, who is our only child, so he can work part time on a different shift than his wife. She doesn't have that option. During the day he is Mr. Mom to their 20 month old daughter and six year old daughter who is in kindergarten. He does the shopping, most of the cooking and some of the laundry and cleaning. By him working part time on a different shift they don't have to pay for childcare for their younger one, infant care is more expensive, nor before or after school care for their older one. That is such a huge savings that all they needed us to do is pay their $100 a month cell phone bill. Without him asking I've paid his $150 a month school loan, before Christmas to help them have some extra money for Christmas, and twice when they had a big auto care expense. Our DIL is a teacher so when the school year ends our son plans to go back to full time. By later August when our DIL starts back to school our younger granddaughter will be over two years old and I have no problem with her going to a sitter or daycare as at that point she will be old enough to talk and tell them if something bad happens to her.
Last September I went up to help our son and DIL out for almost a month while he trained full time on days for a new position. I saw how complicated their lives were with both of them working full time. I would gladly continue to pay their $100 a month cellphone bill and the school loan a couple of times a year to have our son stay part time another year until the younger one is three years old. By then she will benefit from the socialization of being around other children her age on a regular basis. But that is their decision.
The jobs and schedules our son and DIL worked when their first child was born meant that they didn't need child care her first year. After that their schedules changed and they had to find a sitter three days a week. A couple of months later the sitter, who was wonderful, was offered a full time job by a previous employer. The new sitter was someone their neighbor had known when the sitter was growing up and had known her whole family for years. The sitter was now 25 or 26 and married.
The job my son had at that time meant he had to work killer hours from Thanksgiving through New Years. Our DIL is a music teacher and at that time was working at two Catholic schools, both of which had Christmas programs that were her responsibility. She also taught private lessons and was the music director for a Church so had extra responsibilities during the Christmas season with their Praise band and choir. I flew up to help the day the flights dropped after Thanksgiving, staying 3 weeks until the flights went up in price just before Christmas. Our son warned me before I got there that his daughter might not want to go to me as she was belatedly going through the strange stage with other people. I thought this was odd as she had never shown any signs of this before and was one of the happiest, least fussy babies I have ever seen. When I got there she was thrilled to see me. Over the next few days I was shocked about the changes I saw in our granddaughter that had developed in just the six weeks she had been going to this sitter. During this time my husband was able to spend a couple of days with us. He too noticed and was upset by the changes in our granddaughter.
I told our son that I had a very bad feeling about this sitter and how upset and worried I was about the changes in their daughter. By this point I'm having a come to Jesus talk with him and I'm in my take no prisoners state of mind. When I get to that point it has never once in his life gone well for him and he had finally learned that he better pay close attention to what I'm telling him. I had been telling our son when we talked on the phone that the things he was telling me about their daughter since she started with that sitter weren't normal. He reminded me that they had asked other people, including my DIL's Mother and older sister about the changes in a child when they get to the toddler stage. They had been chalking up what they were seeing to that. I only had the one child but my DIL's mother and sister both had 3 each so they were seen as being a more reliable source than I was even though they were rarely around the child and not in frequent communication with his wife like he is with his Father and me. I restrained myself from verbally ripping him a new one because I could see that he knew he deserved it and asked him please, please, please find someone else.
Our son was still working killer hours so asked our DIL to take care of finding a new sitter since she was free for the rest of her break once Christmas was past. Our DIL only picked their daughter up from the sitter so just thought the crying for Mommy when she got there was normal at that age. I could have just kicked her in the behind because I had picked their daughter up from the first sitter just a few months before and she was all smiles and waving bye-bye to that sitter. Our DIL didn't do what our son asked her to do. When our DIL started back after break our son had to take their daughter back to the same sitter, she started crying as soon as he turned on the sitter's street. He figured since she hadn't been there since before Thanksgiving, over 7 weeks ago and she only went to that sitter for about 6 six weeks, that at 17 months she couldn't possibly know where she was. The second day she started sobbing so uncontrollably that our son was really scared. He called me on his way to work half in tears saying she absolutely did know where she was, that he was sure that I was right in saying there was something wrong and they shouldn't take her back to this sitter. He ended up leaving work early to go get her and he had a new sitter by the end of the day. Which by the way was someone a co-worker of his raved about and said her kids loved and he had told his wife to contact. Our DIL wasn't entirely convinced about what our son and I suspected until the new sitter commented about the change in their daughter within the first two weeks of being there. They had never said anything about the previous sitter. Then our DIL felt really guilty and I'm happy to say our son wasn't at all sympathetic and came down on her hard. We will never know what went on but it became very clear to both our son and DIL that something certainly had.
The first four years of a child's life are the most important years of their lives developmentally. If you can truly say that your daughter is a wonderful and loving mother to your grandchildren and you won't go broke helping them out then it is so worth doing so. However adding another child to already strained finances is not a wise decision. That would make three children under the age of four, then add money problems to that. Too many marriages don't survive. I've observed that many of the younger generation simply have no concept of how expensive it will be when their kids get involved in sports, become teenagers, start driving, go to college, etc. Your daughter and SIL are already behind financially and she really needs to go back to work in another year to eighteen months. But maybe they will win the lottery or have someone in one of their families die and leave them a nice chunk of money to get their heads above water before their kids can be self supporting.
BTW our son and DIL never intended to have a second child. They've struggled financially their entire marriage and with their first ready to start school weren't going to have to pay for daycare anymore so would finally start seeing the light at the end of the tunnel. This was their second oops and they are making very sure there are no more oops! Their plan when they got married was to wait five years before they started a family. Two months into their marriage and they were expecting. Life doesn't always go as planned! I do give my son credit as he contributes as much to his 401K as his company matches and has never touched it. Unlike his wife who will get a State teacher's pension, he knows the chances aren't good that he will ever have a pension from an employer.
We lived frugally for the first nine years we were married so that I would be able to stay at home with any children we might have. This was very important to both of us and a goal we worked hard and sacrificed to make happen. My Father was diagnosed with Multiple Sclerosis when I was 11 years old and only had social security to live on but he would have been glad to help both my sister and I if he had been able to work. Neither of my parents had parents who were in any position to help them monetarily. However when my Mother went back to work when I was 18 months old my Father took me every day to his parents house for his Mother to take care of me. This continued until I was about 4.5 and my sister was born. Those grandparents always had a big garden that supplied them, us and my aunt's family. My paternal grandfather paid me for my A's and B's from the 4th grade when we started getting letter grades until he died when I was 16. Of course like any kid I liked the money but that he cared so much about my grades that he paid me and was proud of me was what was really important to me. My father was having health problems by then and my parents were not in any position to reward me for my grades like many other parents did so my grandfather stepped up. He would ask about those B's, not making me feel bad about them, but in a way that made me feel like he cared whether or not I liked a subject or it wasn't as easy for me as other subjects. Not all help comes in the form of money. My widowed MIL was 92 when she passed a few years ago and she clung to her money with the excuse that no one ever helped them. Which was absolutely not true as both sets of parents had helped them quite a bit and she inherited a nice amount from her Father. My husband's two older brothers still talk about when their Father's Mother showed up with a grocery bag full of money so their parents could pay off their house and other things both sets of grandparents did for their family. And I'm sure there were other things they didn't know about when they were little or weren't aware of when they were older. So instead of helping all five of her children when they had young families and could have used the help, money or otherwise, they inherited the money when they were old enough to no longer need the help. Yet during all of their younger years when they struggled to get the bills paid each month their Mother was always quick to criticize that none of them were saving or saving enough. All five of her children are smart, hardworking, none of them into drinking or drugs, or the type to sponge off her. All four of her boys have been good husbands and fathers too. She wasn't interested in her grandchildren and I don't remember her ever praising or rewarding them for anything they accomplished. Although she was thrilled to brag to her friends about anything they did she considered worthwhile. She was very fortunate that my husband's Father thought the world of his Mother and drilled into his children that they should love and be good to their Mother. I would rather die a pauper than be remembered like my husband and his siblings remember their Mother, that money was more important to her than they were.