OK, assuming you've been candid and truthful about the facts of this story (I read that other thread where you weren't), it seems to me like it's your mom's call since you're her son. If she chooses to buy you weed and endorse your using it at home, which I'm gathering is her home and not the jointly owned/supported home of her boyfriend or his loser brother, then that seems like reason why you should be able to do that. I personally find her choice curious since she's putting herself at risk buying it for you, both with the criminal justice system and from a child custody standpoint if anyone ever found out she was your supplier source. I truly believe that minors whose brains and bodies are still developing ought to refrain from smoking weed, but I know lots of folks here believe differently.

Bottom line is if she supports you and is the sole financial supporter of you in her household, then that ought to be the determining factor. Sounds like Andy has a major loser for a brother and that he's a bit of a hypocrite for wanting you to follow no-weed rules that he himself doesn't follow.