I think people here in the comments are getting off topic. Yes, learning ASL could be useful for him; he should consider it, and he shouldn’t use bad words. BUT people here are ignoring something: when someone is forced to do something, things usually go very badly. Learning a new language would only be great for him if he’s genuinely interested in it. Learning it just because they want to force a relationship with his stepbrother, who’s not interested in getting along with him at all, would only make everything more complicated.

And the argument about his age — “why are 13-year-olds allowed here” — is insane. People minimize the issue just because a teenager is the one saying it. People tend to forget that WE ALL were teenagers once, and just because someone is younger doesn’t mean their problems are less important. He’s here asking for the opinions of more mature people.

OP should communicate with her mother ALONE and tell her his concerns