BIL and wife from hell

mo

They witnessed us have an argument and afterward, in private, my BIL told my husband that “if you weren’t already married, my wife has a relative you could marry.” BIL and his wife have been trying to make me look bad since the beginning while trying to portray themselves as the perfect couple. I shouldn’t have to prove myself to them or my MIL/FIL. What would you do?