How can I get my inlaw's dog to stop these bad habits?

It sounds like he is not trained because the owners are lazy and fall short in keeping it consistent with him. He needs to have more commands/training by his owners. If they don't follow through he will never get properly trained. They can hire an in-home trainer (which is probably expensive), but if they are willing to do it, I am sure their dog can become a more well-behaved dog. Good luck.