What a nightmare.
What has struck me most is how much pain and discomfort your father is causing all of those around him.
Your mother and brother, obviously.
You.
Your in-laws.
It'll get passed on to your children too, you know. You'll do your best to shield them - but you won't be able to stop it.
Your father is a huge problem. He may well have ASD - but he's horrifically, painfully controlling of anyone who falls in his path and is unable to escape.
I think the first step is to acknowledge the burden you are carrying. The damage he inflicts on you.
Then you can move on to thinking about the damage for your brother and mother.
You are still in the mode of trying to manage the damage.
You're thinking about strategies to look after your mother - manage the damage in their relationship. Which of course leads to you helping maintain his relationship with your mother and your relationship with them.
Ironically, all that managing actually helps maintain his really awful, damaging control of all of you.
So, actually, I think you need to start thinking, 'What will help me and my important relationships (husband, children)?'
Clearly, the lie you're being asked to keep about your brother is a massive source of stress for you.
So is making excuses for your father.
Test things.
Stop keeping secrets about him from you in-laws. See what happens.
I suspect that, for all the fall-out, it will be better for you to stop.
Likewise, the 'secret' about your brother.
Incidentally, that secret effectively separates you from your brother. And severs the relationship between your brother and his mother.
It effectively diverts all these relationships through concern fir your father's choices.
It's power and control.
No wonder it makes you feel stressed.
Your father comes first, his choices separate your birth family, and there is a constant threat of 'punishment' (of your mother) that keeps all of this in place.
It's horrible.
You poor thing.
20 years of that.
No wonder it's stressful.
I think it's amazing you're here, thinking things through. It's a remarkable step to take - seriously.
Good luck with whatever you do.