I have a question.  I've never had neurochemical problems like PPD.  But personal experience has been that my mental well-being is muxh better when I have non-familial, IRL validation that, no, I'm not a bad person and they're just crazy.  That factor helps a lot when you're an atheist.  Would that help in your case, or does depression render external positives irrelevant because of anhedonia?  It wouldn't fix the neurochemical problem, but would help alleviate the in-laws problem.