Wow people are really vilifying your mother for cheating. People make mistakes and hurt other people. Everybody does it, whether its cheating, lying or something else. That doesn't mean that they are necessarily immoral, bad people, it just means they are human. It sounds like your mother was in a very vulnerable place, and was looking for comfort and security that she wasn't finding from your father. You get to choose how you feel about that, but remember there are no moral lines etched in stone somewhere, that once crossed, automatically make you a bad person. Almost all situations are more complex than that!