There's a traditional sort of idea that men are supposed to "contest ownership" of women, which gives antagonism between male relatives and male romantic partners of women a different sort of shape than antagonism between female relatives and male romantic partners? At least, that's why it might tweak me a little.

... but only a little, because sometimes people do fall for people who are bad for them and their loved ones hate seeing it, regardless of the loved ones' demographics.. I am not sure if I would recommend the LW be miserable for an afternoon on their sister's behalf, but this might be my own life experiences talking: if I were advising the LW I would tell them to make sure they have someone to sit with and quietly gripe with at the wedding, if at all possible.