Your mum is not acting in her son's best interests. It's worth pointing this out.
My uncle lived with his parents - but managed to work for a few years before they moved. He's two years older than me as there's a big age gap to my Dad (they are half brothers)
His mum did everything for him. He really needed an autism diagnosis but his parents refused to do this. He's not incapable though. He just was never taught various life skills.
When my Dad's step mother got older and started struggling with her health, my Dad did persuade them to name my uncle as her carer. Even though the reality was she was really doing most of the caring for him. This is so he would get some NI credits towards a pension otherwise he would have absolutely nothing.
She died not so long ago and it's been a car crash. There's 2am phone calls to my parents for smoke alarm batteries dying. He can sort it out as he's capable of doing so - he isn't particularly disabled, he just has no common sense or idea of any real life skills because his mother did absolutely everything and he had no need to learn. It's so sad. My Dad knows he has to fail to get the support he ultimately needs.
The situation you are in is more difficult. If they do move so close and your mum dies he will make your life a misery through harassment and manipulation. You need to talk to your husband about this. You may need to move and you may need to consider going no contact or even take out a harassment order to protect yourselves from him.
And this is what I would actually be actively telling your mother what will happen. If she wants you to have a relationship with your brother it has to be a healthy relationship - which includes him taking responsibility for him. You need to be blunt that she is actually harming his future, because once she's gone you will not be picking up the pieces and you will not be emotionally or financially abused by him. She is putting him into a position where he will not be eligible for a state pension too - even if he has his own house, this is going to leave him desperate.
If he is too ill to work he needs to be assess accordingly, so he can get his NI credits. If he isnt that ill he needs to be actively looking for work and claiming job seekers whilst he does this in order to get his NI credits. Not doing either and her just supporting him financially is her actively harming you both and setting up a future conflict and for him to be in a position where he will be at risk from her actions and will struggle to make ends meet.
She is effectively financially abusing you both. And she's certainly emotionally abusing you.
See it for what it is.