Sister in law trouble! Help!

So I've been with my husband for 8 years and never had a problem with his sister...but my problem is that whenever we go to mother in laws house (SIL lives there with her 9 month old) we never know if she is going to be in a good or bad mood.  Every time we are there she is yelling and screaming and pissed about something.  so today is my sons 2nd birthday and once again we walk in the house and within 5 minutes she is  starting with her attitude and raising her voice at my husband, so I told my son to come play outside and she turned and started yelling at me and I finally spoke up and said I think it is inappropriate that she is always yelling in front of the babies. And she flipped out so I left.  I apologized to my MIL for raising my voice in her house. But still idk if I should have said anything. I am just so sick and tired of her always screaming at everyone every time we go there.  She lives there and eats there for free and is extremely rude and ungrateful. Should I just avoid her for a while? We usually go there on Sunday's for family dinner but I don't even know if I want to go now.