(((freewill))),
I agree that you are under no obligation--nor for that matter is the bride's family, in my opinion.

I will just tell you what happened with my parents and mother in law who were all on fixed incomes when I was getting married.

When I got married my husband and I paid for a small, very modest wedding. We had no expectation that my parents or his mother would contribute. They did give us checks for $1000 and $500 as wedding presents which we considered very generous given their financial circumstances.
Your situation seems more difficult since the relationship is strained already. Good luck with whatever you decide.

Meta