Just my opinion, but since she approached your husband first, I really think he should be the one to tell her no and stick with it. Have had similar situations too with my in-laws.
Then, if she comes to you next (and sounds like she will), you can just say that you and your husband have already discussed it and that you are planning to use those miles yourself. If she asks what you are going to use them for, can say "oh, just plans to spend some time with the family." If she keeps on wanting specifics, you could say you don't have any specific dates in mind yet, but that you are definitely going to use them.
She sounds like the type who will come back time and again trying to get those miles from you. Just tell her the same thing every time. Finally she WILL get the message. And at that point is where she could possibly exhibit some anger. Because it sounds like she feels entitled to these miles. If that happens, then it is hubby's job to tell her in no uncertain terms that she needs to stop asking, that she is not entitled to them.
Also, there is no way you can look up her airfares for her without her sitting right beside you saying yes or no to different results you find on the internet.
Just tell about some good websites that she could look at and let her do it herself. It sounds like this woman has been babied far too much - she acts like a petulant child instead of a grown woman.