You have done the right thing posting here. You cannot raise your DCs in a toxic environment - they will absorb and internalise everything including your unspoken frustrations, anger, hurt, despair - this will be very confusing and distressing for your DCs and will impact their emotional development and stability.
This situation has to end.
How and when is the detail.
You should investigate all your options privately about separation. Be clear what your rights, options, choices, timelines processes etc are.
Once you have this information even if the alternative is not as financially or logistically comfortable as you are presently (assuming that there is some financial slack as you must have a 4 bed property now) - your DCs emotional future (and yours) will be much richer and stable by living in a calm and peaceful home.
You just need to know that the current situation is not sustainable for you and that there is a way out - when the timing and arrangements best suit you.
Once you have done all of your research you could then approach your DH with the following 3 pieces of information.
- A requirement to move his DM and DB out.
- With a clear deadline.
- A clear consequence if this deadline isn’t met - you will be divorcing.
Ignore any banter about custody that’s just more bullying threats and bluster and carry on with your well researched plan.
Maybe he is angry because he is aware of the toxicity and himself feels trapped and overwhelmed by them and culturally. If he could admit to that you have a chance of saving your marriage. If not you need to take responsibility for moving your DCs out of this mess as soon as is possible for you.
It must be overwhelming to consider single parenting but it will be much easier than your current set up.
Seek professional emotional support though low cost counselling or charities to keep you on track. Do you have friends IRL to lean on?