What should I do if my parents fail in business and are in debt, and my father-in-law looks down on me?

You can refute tactfully, never contradict. If you don’t want to be looked down upon, then prove yourself. If he cares about you, he can see your value. Of course, it does not rule out that some people are snobbish or confused. No matter what, you should do what you should do. You are my son anyway, so just have a clear conscience.