Maybe you should set up a time to meet her for lunch and talk with her about why she seems to resent you, etc. There is plenty of time between now and next September to work things out, and if you still don't get friendly with her, ask her if she would be willing to do a reading or something, rather being a bridesmaid, that way she can be a part of her brother's big day, but not have to hang around with you very much.