Maybe she won't be there. Or does the group sometimes split off into two groups if it's large? My local DBSA usually does. You could go to the other group than her.

If you doubt she has bipolar disorder maybe things she might share will change your view on that. Or maybe if she isn't she'll get a better idea of the realities of the disorder. Maybe she is experiencing some strife and looking for answers. I hope if she comes she benefits in some way. Perhaps you'll grow more comfortable with her, or her visit will be a one time deal.