Totally agree
Actually as family you need to learn that overprotecting is what make a person not only feel but to actually being disabled
Cause OF COURSE there're certain disability that may required more help from others but you need to understand that at some point you're not always be there and that they're and need to be independent adults
And honestly his family has already done enough by learning sign language. When most the times the family with ostracize them or make them conform to being "normal" like forcing them to get a cochlear implant. Or they wouldn't even spend the time learning to sign at all.
At least his mother wasn't so against it. She probably just needs time to accept him wanting independence. After all he's still her baby and of course any parent would worry for their child who has a different set of circumstances against him when it comes to surviving in the world. The brother needs to learn to be less overbearing. Sheltering him is only going to make him feel even more disabled like you said.
I feel like in stories that involve the MC having a disability their siblings can either be over protective of them or they're complete assholes and disregard them as a whole.
I get it's tough navigating the world being deaf but the brother needs to let him learn how to do things on his own. They can't always be there for him. Being independent will help him grow and understand his limits of what he can and cannot do.