Family Christmas? Every year, we, as a family, my sister, brother and all our families get together....?

And this year my sister decided to pull the plug because she is upset with my brother. I know the reason, I don't agree with it and have told her I didn't. No one feels the same way,except for my sister. She refuses to go if someone else holds the party because my brother will attend. My bro called sis and asked about this Christmas party that he thinks is still on, and says nothing to him. Her answer was "I don't know" He calls me, and for now, I play dumb. I am really upset with her for making me lie about it, and for her to pull the plug and she doesn't tell him why. I am so ticked, I know what I WANT to do, but what would you do? I want to call her and ask why I ended up lying to him for her then call him and tell him what I know. There are 28 family members that go to this family party, the 3 of us kids, our spouses, kids, and grandkids. There is no get together, she pulled the pin, she won't tell him why, and denies knowing. I am stuck in the middle, and I hate being there.