Help! Need Advise. Family Drama.

Are you and FI hosting or did you only help with the addresses? It's inappropriate to host a gift giving event in your own honor, so if that's the case moving it somewhere else is not the right answer. If it was your future in laws are hosting then they have discretion over the guest list, certainly in their own home. In that case, either accept the fact that FI's aunt is not invited or cancel altogether unless someone else offers to host.

Find a new venue or uninvite the aunt. Those are really the two options short of cancelling the shower all together. And this isn't an issue for you to involve yourself in, other than which decision you want to make.

It's their house, so it's their rules. Unless your finance's mom can convince him to allow his sister to attend, you may be out of luck, but again, that's not something you need to be involved in. All your fiancé can do, is talk to their parents, voice their wishes and accept the result.

Some relationships aren't as black and white as just be civil.