Advice for dealing with my adult autistic brother

My brother was late diagnosed with autism and also struggled with depression for years now. He's been in a mental health facility several times now but he falls back into old patterns immediately after leaving it. He could get a lot of support resources otherwise, but declines it as he can "do it all by himself" (except for meds).

Instead he relies on the support of our mom. Financially and physically, our mom needs to save him regularly as soon as his tries for independence (moving out into his own apartment etc) don't work out again. He treats her like shit, takes her hard earned money and kicks her out of her apartment because it would be too 'uncomfortable' for him when she would life with him together in HER apartment. Our dad is out of the picture.

She is close to a breakdown herself due to the endless struggle with him, that's why my husband and I offered that he can life with us for a while, also because we have a very structured life that helps him to cope with depression and autism. It seems like he didn't really plan to stay with us. We asked him to take out his trash and let him do his thing otherwise and after a few days our mom visited and he screamed at my husband and I that we are too controlling and he is an adult who doesn't want to be controlled and our mom should take him to her apartment. Our mom granted him his wish, saying she needs to save him from us. While living with us, he took a lot of sedative pills and as he ran out of it, everything escalated. We noticed several addiction signs during his time with us.

I love him and our mom deeply, but I don't know how to help him or her and it's hard to watch him abusing our mom emotionally, verbally and financially for years now (there was no physical abuse). I know he deals with a lot of issues, but they will probably stay for a long time and once my mom is older I will not accept him abusing me in the future as well.

Is there any advice on how I could help them while keeping my boundaries?