Op. You've got 2 choices really here if the current situation isn't what you want (realistically this choice is going to be forced through anyway due to money running out, so it's a WHEN not IF situation).
You need an honest conversation with DH.
Sit him down and explain that you love him and have supported him, but you will not accept this situation going on until it drags your family of 3 down with them.
Give him a choice.
Option A) Does he you to get hands on and support? Appoint an estate agent, get a valuation, get the legal side of things kicking off so that the courts will force the sale of the house, which should have happened 10 years ago. Write off the rent the siblings owe him. The condition of him accepting A is that he stops impeding you. This situation is affecting you and your child, if he chooses this, he needs to go all in and accept the likelihood that his siblings will get vocal and cut him off. He'll take this if he wants to sort the situation but out of his depression he can't see a route forward because he can't face dealing with his freeloader siblings.
If he doesn't want to do A, he's made his choice - his siblings over his own wife and children.
You can therefore assume:
Option B) divorce, to start protecting yourself & child from his emotional situation and financial downward spiral. You might say you'll continue to support him emotionally but legally you need firewalls.
It really is that simple op.
Write him a letter, short, if needed. But don't look back in 10 years and realise you've been working to bail all 3 adults out.
Your child deserves someone to step up and be the adult here, because the other 3 "adults" are basically burying their head under duvet hoping the problem will go away by magic.
Your husband depressed doesn't change the fact that you need to protect yourself and DC. It's a simple as that.
Don't let the emotional guilt tripping convince you otherwise.