I'll go first.
I wish I could tell my SIL that I dislike her so much and I wish she'd never met my BIL. I hate that she is now part of our family. She came to disrupt our (far from perfect but lovely) family. I hate the way she treats my BIL. I hate how she makes everyone pander to her every whim. How all her messages are passive aggressive and how rude she is to my MIL. I could never say this out loud of course, but I dream of it often.
Feels good getting that out of my chest.
Anyone else feel free to add.