@David, Moderator posted:
There, Jacob, it is not the father's beginning to yell at the mother which escalated; rather, it is simply his yelling at the mother. Because "which" refers only to the verb phrase "yelling at the mother," the "which"-clause is not a sentential relative.
I guess I'm not understanding how sentential clauses (or relative clauses in general) work. 😳 I'll try studying this more.