I am had the same sort of dillema, except my fiance asked my brothers to be in the wedding party which made my situation even harder. My fiance's sister is loud, immature and can be very obnoxious and all I could keep thinking is she is going to be at my bachelor party, bridal shower and all of the other events anyways. She will be part of my family very soon and I just thought it was the right thing to do to involve her. I think it would be a very civil and decent gesture and maybe even a good time to become closer with your soon-to-be sister-in-law. However, don't let her ruin your day.