My son was in an accident and is now a quad in a power chair. The family was in a great deal of stress as he had to be taken care of round the clock. He had to be catheterized every 4 hours. He was in and out of hospitals every 6-10 weeks with urinary infections
We found that we eventially had to create a situation where he was admitted to a nursing home and then stayed there until all the Medicaid home services were in place. Then he came home to live with daily services taking care of many of his needs. This took a great load off the family. Eventually a supra-pubic was put in place.
All this had to be done before he came home, to have the services he needed, taken care of by medicaid and other services.
If all your Dad has is his SSI or SSDI, he should qualify for many different services and programs. You need to contact an assist service to help you out. Legal Aid can steer you in the right direction. So can SS.
Also, if your sister-in-law doesn't work-she should donate some of her time to helping her father-in-law. After all, she's now a part of the family. Sounds as if your brother and his wife are being selfish and not handling the part of the load that they should.