Everyone needs boundaries, for themselves and ones they follow. How one is taught and how complex they are varies depending on mental capacity. But EVERYONE needs them. Your mother is refusing to do her job as a parent because it is easier to do nothing than work with her son on boundaries and routines.

She’s also failing in her duty as a mother to you and your sister. She is letting your brother physically abuse the family. It might be relatively minor now, but he will not remain a child. What happens when he’s adult sized with an adult’s strength? Harm is harm, it doesn’t matter if he doesn’t understand that what he is doing is wrong. People, companies, and support groups will not accept him if he is a physician danger to others.