Oh my god, that is horrible. I wish i could punch everyone who has hurt you. You are only 15 and should not be going through this.
The best thing you can do is to forget about your real mom. Even though she’s your mom and your blood, that does not mean she’s a good person. Friends are family you choose for yourself - just because she is your mom does NOT mean you are obligated to her what so ever.
Your mom wanting the checks is horrible but maybe the further you are away from your mom and parents, the better for you. You can have a happy life without them ruining anything.
Since your adoptive father molested you, i would go to the police right away and tell them. He is a horrible man who deserves to be behind bars - you owe him nothing and have no obligation to him either.
If you don’t want to talk to the police (but i would strongly recommend it considering how serious this is) talk to your guidance consuler or a favorite teacher about this and then you will get the help you need (the police will probably get involved)
As for your brother keep away from his sick ass. Carry pepper spray with you so if he ever tries to touch you, spray that on him. You owe him nothing and he’s a sick bastard.
Honestly, i would just get the hell out of there and tell the police everything. That is your best and safest bet. Don’t think “i cant handle it’ or that it isnt that bad because it is. or that you are strong enough for this because this is not how it is suppose to be. You should go to the police right away. Don’t even think for a second that any one of them will change - they are sick and horrible and you do not deserve this.
You can always message me off anon lovely, and you can always talk to me. Please tell me how everything goes.