My moms totally unfit to take care of him and is threatening to send him away to some home is there any way I can stop it and adopt my brother?

Heather, how old are you?
I don't want to give you incorrect information. I've only seen such movies based on true stories where this did happen so please, sit tight and the people here will help you. I want to follow up myself with this since it is very sensitive. I will check back. Hang in there and my best to you.

Your age is going to play a huge factor. Your current status (student, unemployeed, gainfully employeed and supporting yourself) is also a deciding factor.

As for adopting... possibly not. There is always a solid change of gaining guardianship however.

I would concentrate on getting guardianship. Go to your local family services agency and explain the circumstances. If you can show you can support both of you, I think you stand a good chance of becoming his guardian.