Major MOH Problems - Please Help!

I'm having a major issue with my sister, whom I've asked to be my MOH. I also asked my lifelong childhood friend to be second MOH. Apparently this is totally unacceptable to my 19 year old sister who is throwing a fit over not being the only MOH. She thinks there should only be one MOH and not 'Maids of honor.' I really don't know how to resolve this without hurting anyone's feelings. I care about both of them and truly want both of them to be my MOH's. But this is causing so much drama between my family because my sister is unwilling to accept my choices. Neither of them are married, and was wondering if there is another title I can give my best friend that isn't MOH, but not just bridesmaid! My sister said I have to chose her or my best friend otherwise she's not going to be in the wedding.