I am sorry to hear your story. You can firstly talk with your boyfriend about the issue again until he realize that the issue is rerious. Then you may ask your boyfriend to have a talk with his sister. By doing so . the relationship between you and her may be better off.
Perhaps it makes sense that your sister in law should be in the bridal party, but I think that it's not just your right but an obligation to set the boundaries from the beginning (both you and your fiancee). It's your wedding and you guys have the last say on all matters.