"Why does he always have to point that out?"
That is a good question as it doesn't strike me as odd that your aunt and uncle have a passion to help deeply wounded youths. I can see how it could be confusing and troubling to hear your dad joking about a subject which is solemn. If your comfortable sharing more, I don't understand what you mean by 'bad boundaries and red flags'. If your not comfortable sharing more, that is ok.