Its so hard to decide these things. I also didn't want to hurt anyones feelings, as I have a couple very good friends that wanted to be the MOH, but i have opted out of the MOH thing altogether.I've decided to have only bridesmaids, that way nobody gets mad and everyone is equal. I'm not saying this is right for your situation seeing as how its a bit different. I do think that maybe asking if she wanted to be a bridesmaid is a good idea. Is she the type of person you could really confide in, because if so i would definitley tell her of your reservations...i would with my sister.
Sorry I couldn't be more helpful. Good luck!