The sim I currently play with, Brooke-Lynn; her father cheated on her mother and had three children with another woman in the neighborhood (1 single birth and 1 set of twins). Her mother Mackenzie was pissed at him for a long time and even started seeing a younger man at one point. Eventually, she broke it off with the younger guy and after sometime her and her ex-husband reunited and got married again. The second time around was MUCH happier and Brooke-Lynn and her sister, Lacey, got a new brother out of it.
I got bored so I had him cheat, and my game got really interesting after that, because they'd been together since they were teens! I didn't cause problems in their marriage until he became an adult (and I play on long so they were together for a long time).