I told Mother-of-my-Children what Youngest Son said about arguing all the time with Oldest Son when they are at her house. She confirmed that they argue a great deal. She said that Oldest frequently answers "no" when Youngest asks her if he can do something. She thinks he is trying to be the man of the house.
That seems plausible to me. My Beloved Wife and I were talking about this the other night. Oldest would not try to take a pseudo-parental role in our house because A) we have clear structure and rules at our house, B) I am there to be the man of the house, and C) he knows that I would smack him down hard if he tried act like one of the parents instead of one of the kids.
Based on what happened when we were married and my observations now, MomC does not maintain clear structure and rules. So there is a vacuum that Oldest is trying to fill.