The problem of you is that neither of you were really wrong, but then neither of you were really right either.
Don't conflate this argument into her family always coming first. Fight fair; this fight is about this fight
Her MIL came over unannounced and at a bad time, damn, **** happens. I wouldn't be happy about it either. But people are people and it happens. I wouldn't expect my wife to turn her away at the door.
I wouldn't feel the need to interact, but I would be sure to interact *some*. If not, you look like you're mad and stewing in another room. Drop by once or twice for water or something. Find an excuse to see them and chit-chat for a couple of sentences to show that you're not steaming mad, but keep it shor so that it is obvious that she has interrupted something.
Your wife should realize it's an inconvenience and it's perfectly ok for you to feel like it's an inconvenience and understand that you don't want it to be normal. You should realize that, well, sometimes your inconvenienced, and make an effort to not be mad about it, because well, life and all that. The best laid plans of mice and men....go often askew.
My wife is easily on the phone 30 minutes with her mother pretty much every day. Add in the time with her friends, adopted family, others, etc there are times where I don't even really get to talk to her (at least their used to be). We've talked about it, and she pushed back for a bit, as did her mother (I'm you're mom!), but the bottom line is that I need more than 30 minutes with my wife each evening. It's not ok for me to have to do everything around the house because she's always on the phone, and it's not ok for me to be her chauffer because she thinks that car rides are free time to catch up with people. But it took a while of bending but not breaking and holding firm to get her to realize that she doesn't need 2+ hours of family phone time every evening at the expense of talking to me! Little fights like this were common, but it gets better if you fight fair and do bend adn flex some here and there, but stay consistent