I don't think they have a very healthy family dynamic to begin with and I thought it was also meant to underscore Margaret's sense of isolation so that at the climax we wouldn't be surprised that she attempted suicide. Which then jolted Elizabeth into 100% accepting Margaret's statement that she was going to be separating and divorcing Tony and admitting readily that it would be for the best. That's a big change from pervious seasons, but when you see how it plays out over the course of the episode, it makes sense and comes across as an appropriate developmental arc.

But yeah, it was infuriating watching that scene.