Bachelorette party

Some people do invite their other friends to the bachelorette parties. Friends and family not in the bridal party may want to celebrate, no one should feel weird about who's invited. The purpose is to celebrate you and your upcoming wedding. That said your sister should understand that the others may be upset if her wife is the only person not in the bridal party. Unless your SIL is a a trouble maker I would let your sister handle this, since she is planning the trip. No one can be upset with you when you did not plan it. You said you're not sure how the others would feel, but don't stress yourself with the what ifs, they can speak up if need be.

Well...I would talk to your MOH and tell her that you are ok with your SIL coming provided that she be sure that they both spend time with all the bridesmaids and not isolated. Also, I think it would be nice to invite the other future SIL to at least make peace since she was hurt not being asked to be a BM. She probably gets you two are not close but I guess it is always awkward to invite one and not the other.

If you want it to be just your bridesmaids, then stand your ground and have just them. If you want to include more (like her significant other) then I'd invite other friends like your fiance's sister.

Agreed.

She is hosting the party and while you do get a say in the guest list, ultimately it's up to her to plan and organize the trip including the guest list. If anything, I would talk to her personally with your concerns as to why you think her spouse should not be there and then the two of you come up with a solution. I do understand a couple at a bachlorette party is not the norm, it does happen and maybe she doesn't see the angle of issue you are seeing.