That's a tough spot to be in. Ultimately, it is your and your fiance's day and no one else's. You may want to hold off a bit, but if your gut is telling you to have your babtsitter/best friend as you MOH, then go for it. Could you have her as your Maid of Honour and your sister as your Matron of Honour? I've seen some people do it this way and it seems to go well. You may also want to have a private conversation with your sister to see where she is at. Who knows, she may prefer to not have the responsbility of MOH... Good luck with everything