Sister in laws...what should I do?

I have two sister in laws that I cannot stand. One irritates me to no end...at Thanksgiving she said to my stepson "no one will EVER be as cute as you" right in front of me and my 1 year old son. She's lucky I'm not the type to cause fights because I wanted to say "listen here you prissy, stupid, uneducated, worthless b****:...what would you do in this situation? She is CONSTANTLY bragging up my stepson and pushes my son out of the picture. Should I FINALLY say something or be a bigger person and let it go in one ear and out the other?