L.W.
Oh Lordy...the Shower Drama. I am telling you that happend at my shower and then it just happened again at my friends shower. Everyone want to throw the shower but noone want to do the work.
Personally I would cut through all the BS and throw your OWN shower and invite all of them. You can tell your MIL that you are having a sister family issue and think that if she threw the shower it would cause more stress on you than it would help. But tell her she can help plan the shower with you. If you sister gets mad, well then that is her choice. i would tell her you decided to do your own shower becaue everyone was getting you all stressed out. Sadley you should not have to throw your own shower, but to save some heartship down the road, that is what I would do.
If you dont like that idea, you can just confront (nicely) to your sister and say "hey I heard that because you are mad at me you dont want to do the shower..is it true?" see what she says. if she says yes then just tell her that MIL is going to do it.
Oh I HATE it when something nice and fun turns into a soap opera. Good luck. Its got to be frustrating for you. Hang in there though.