Once we found out about this, we weren't really surprised that Boo was so ready to fall in love with Sully in 'Monsters, Inc.' It was probably a welcome distraction from what appeared to be a complicated situation in her home life.
It's only visible on screen for a moment, but look at the picture that Boo has drawn of her mom and 'Uncle Roger.' You don't win any prizes for guessing what they've been up to, but there's no way it should be in a kids' film.