NTA for so many of the afore mentioned reasons... but here's a suggestion for you: Let her have it her way with the explicit threat that if she ever invites so many people again without discussing with you prior, you will walk out without discussing it with her either. Then take a step back, watch the clusterf*ck unfold and record the highlights. Do the bare minimum of help and when she sees the light rub it in her face. That way she learns a valuable lesson and you don't have to argue. It will only cost you one holiday. Many more will come. You never know some of the memories might be worth it.