ESH... I think his closeness to his sister sounds weird, but if he’s got such a strong bond with her, then of course he’s going to be upset if you try to ban her from the wedding. It sounds like there’s a deeper issue here that you guys should consider trying to work on before getting married. I don’t think that banning the sister from the wedding will do anything but make your fiancé and his family resent you. Also the sister (who is still a child) isn’t even the problem here, it’s your fiancé.

He sucks for not addressing your feelings about his relationship with his sister more. He should be listening to you or at least trying his best to make your understand their relationship more.