I'm going to refuse to help with my sil's birthday party preparations and during the event when she gave one day's notice
Wow, is she doing anything herself?
She didn't give me enough of a heads up and she does nothing to help me with anything!
It sounds like maybe she just forgot to mention it?  You should be the bigger person here and help her out, since that's what in-laws should do for each other.  Hopefully she pays you back by helping you in the future!
I think she wants it to fail to try and blame me.
I think that is bit of a stretch to think she is doing this to make you look bad. She could have just been busy at the time and totally forgot. Try to help her if you can.