I watched The Other Woman (2009). This is the Natalie Portman film, not the Cameron Diaz rom-com with the same title, nor any of the 20 other Other Woman movies on IMDb.
At the beginning of the story, Natalie Portman’s baby has recently died. As bad as that is in itself, the baby was also going to cement her relationship with her husband, which started as an affair. Now her only hope is to ingratiate herself with her husband’s son, but he’s weird and insensitive, and her husband’s ex-wife is trying to sabotage her. I liked the movie enough to read the novel it was adapted from, which has a much better title: Love and Other Impossible pursuits.