I'm the same hun, I wouldn't say it's "sad"- I've moved around a lot in the past few years & the girl I always thought would be my MOH turned out to be a complete manipulative cow & I wouldn't even think about inviting her now OH is my best friend too & the only other friend who I am extremely close to is a guy
I'm having my sister & SIL2B - my SIL2B has been a bridesmaid about 10 times now so she is a veteran so i'm hoping she'll be extremely helpful!
Never too late to "sack" them anyways, as cruel as it sounds - with your SIL you could just say you think she'd enjoy herself better if she could be with her little boy & your sister you could have as a witness but not your MOH. These days, it isn't unheard of to not have any bridesmaids at all, which might be less hassle for you, as well as being cost effective. Do you have any nieces or young cousins that could be mini-maids for you, instead of having adult maids?