My autistic brother just admitted to something horible and i dont know if i did the right thing?

Yes, you did do the right thing. And think about this, your brother is autistic, a very serious condition, and he doesn't know what he's doing, so by telling your mom and taking him with you, not only will you help him, you will help your mom because she needs you. You will also teach him to interact with others better. I hope you take this into consideration.