Your maid of honor traditionally is the one to throw your bridal shower. The fact is that you chose her to be your right hand woman throughout the whole wedding process. If she wants to throw your shower, she should probably be allowed to.
On the bright side- this may be a great opportunity for your best friend and your sisters to make a truce. If they have to work together for a common interest (you) they may be able to look past their differences, and even enjoy it. If I were in your shoes, I may ask my maid of honor to call my sisters up and ask for their help. She will probably understand the fact that your sisters would like to be involved.