I doubt if any principled attorney would see this as a legitimate case enough to try and make it a court fight. Simply because the situation happens so often that it is a cliché. As soon as an older sibling ages out and escapes from a bad situation, they immediately want to turn around and liberate their younger siblings. No court is going to give you custody as unstable as you are at present, you're one degree removed from moving back in with these same "unfit" parents yourself, and now you want to get legal custody of your younger sibling? No, it's not going to happen.
What you can do is as has been suggested, enhance your relationship with your sister, see if you can persuade your parent to let her spend time with you, get her a phone to call for help with, encourage her, be involved with her and appreciate what she is living through, as no one else really can. But don't draw a line and strike up a battle that will alienate you from your mother, and possibly get it to the point she forbids her 12 year old to have anything to do with you. This is a fight she will very likely win. Don't have it. No amount of dirt that you can come up with based on what happened to you ten years ago while you were living with her is going to have any sort of affect on how the courts look at the situation now.
I actually have seen a situation this where, as teen grows up, coming to stay with older sister who has raised me anyway, guess what? Baby sis turns up pregnant. Guess who is father? Older sister's --well in this case it was husband, I guess you'd have to say "fiancé." Suffice it to say this custody arrangement not going to happen legally, and you do not need to go further than a legal consult to find this out.