Sitting back and looking at it objectively, here's my opinion. Nicole was the true guest. MamaChantal confronted her for what she was told by Chantal (not something that she, herself, saw) - that Nicole had clapped when Chantal stomped out of their apartment. Nicole speaks VERY little English, and Pedro was the only other person in the room fluent in Nicole's language. Plus, Pedro would (understandably) want to be protective of his sister in what looked like it was becoming a hostile environment.
So he told his sister and Chantal to change seats - likely so that Nicole wouldn't feel surrounded by a family which was attacking her, and so that he could translate if necessary. What's the big deal with letting them change seats?
Because MamaChantal is the HBIC, and SHE says they are NOT going to change seats. Then Pedro started to (literally) stand up for himself and his sister, and it all went downhill from there.
I agree with @iwasish. MamaChantal was itching for a confrontation, and she got more than she bargained for. The only reason River's lamp swing didn't hit Pedro was because the cord was too short.
It was all ugly, but MamaChantal started it.