There is a sub for situations like this that might be helpful: r/JustNoMIL.
I don’t know where you’re based, but if you live in the US, you should be able to get Medicare to cover some of your costs if you have him apply for disability/SSI. Medicare will also cover caregivers to give you some relief.
As a nurse, I can tell you caregiver burnout is real. It would benefit you to try to get some assistance. If I can be of any assistance, you can PM me! Best of luck to you!
You can put boundaries in place to protect yourself while still facilitating her access to her son. When she calls, texts, or strikes up a conversation about anything other than confirming logistics of when she’s visiting him next, you don’t have to engage. If she resents helping out financially, I think she does have a point that it’s not her obligation to help out in that way. If she’s in a position to and wants to, that’s great but he’s an adult and not her dependent. If he’s permanently disabled, he’s your dependent now.