Absolutely this.
Having someone for a weekend in no way compares to full-time responsibility.
Your MIL sounds quite selfish in that she is insisting your partner and you sacrifice your future to be 24/7 care for your BIL.
The enormity of the ask and presumption for you to never so much as be able to go out for a few hours, without finding care, is really huge.
How do you think that will impact your child and his future.
Your MIL should be availing of respite to help prepare your BIL for the fact respite will very likely be a part of his life, as will a care facility.
You sound like a very kind woman but very naive to what you are being presumed to take on.
Every school run?
Every match?
Every party?
Not a single moment not managing his brother and his challenges?
I have an old school friend who's older sister had a very very slight learning disability.
She was never allowed to leave the house without her sister in tow.
It made life tough for her, friendships very tough.
Not a walk to the shop could she do without her sister who was 3 years older.
Her sister was both a bit bossy and demanding and the whole family revolved around her.
My school friend told me once that there was indeed a clear expectation that her sister would live with her forever, and her parents went on and on about it.
She told me it wasn't happening and she was leaving once she qualified.
She took a job two hours away the minute she qualified as an accountant, even though she had her pick of jobs in the city.
Her parents were stunned and angry.
But she was gone.
She met and married two hours away too.
Her sister and parents got on with it as they had to make another plan.
Unlike your BIL she actually was capable of living independently and she has her own apartment in a little community with a warden, close to her parents and lives her life very happily I believe.
My old school friend whom I haven't seen in years was put in an awful position and her childhood was absolutely blighted by never being allowed a moment to herself witout factoring in her sister.
Be very careful about what you casually agree to.
This situation today will be your future 24/7...constant compromise and negotiation.
You can support your partner without agreeing to this 24/7.
Wishing you well.