Can my mom send me out of the country even if im 18?

hi was born in brasil but im a u.s citizen my mom is american im about to be 18 in 1 month she says that if i move out the house when i turn 18 she will send me back to brasil because she made me a citizen i dont know is that right? she also says that she could do that because im not over 21. and says that if i move out or scape it would affect on my sisters papers because shes trying to bring them. i dont know anything about that please help me