Paragraph 7(k) from the Interval Terms and Conditions says:
"(k) Membership in II may be used only for personal and noncommercial
purposes. Any other use of membership benefits may result in the
suspension or termination of a Member’s privileges, as well as any
existing Confirmations and loss of fees associated with all II
memberships and Confirmations held by such Member."
Renting an exchange is considered a commercial purpose.
Frankly, if you're renting a unit to your brother-in-law and he's reimbursing your maintenance fees, exchange fee and guest certificate, you're probably never going to have a problem. If you're renting out exchanges on ebay or Craigslist and you get caught, they will close your account and you'll use any weeks you have on deposit or exchanges you've made.
Is it fair? I suppose you could say it's not. But it's the rule you agreed to when you bought your membership.