My wife has had a tumultuos relationship with her sister who we think wants her to be the MOH for her 2nd wedding. She was never asked if she would be the MOH, but from conversations they've had it's apparent that her sister is assuming she'll do it. She does not want to, however, I've told her that since she hasn't made it clear she doesn't want to be the MOH she should do it more to keep peace in the family than anything else. If you're wondering why the sister doesn't want her best friend to do it; it's pretty much because she only has a few, she's only close to one of them, and she has already said she really doesn't even want her in the wedding since she's heavy. We think the only reason she wants my wife to do it is because we're pretty much the only ones on her side of the family who throw parties that don't include the words B.Y.O.B. in the invitation.
I'm not sure what your question is. Also I think that you meant that you think she should do it to keep peace, so we will go from there.
I suppose that on the face of it, she would want to be her sister's MOH, especially if there is a possilbility that her sister will be upset if she does not. Plus, it just seems as if she should because she is her sister. However, because of the "tumultuous" relationship the two sisters have, this could be a very stressful time for all involved. This is something to consider. No one has to be a MOH.