She has thought about the effect your coming out will have on your brothers, and HER (interesting), but she has not at all considered the affect that remaining closeted has on you. And don’t kid yourself – it’s unbelievably damaging. No parent in their right mind would ask their child to hide their true self, to do something as painful and unhealthy as be a completely fake person for everyone, 24 hours a day. It’s utter insanity. Your mother was absolutely wrong to tell you to stay in the closet. She is supposed to be your rock, the most supportive person in your life, and she has selfishly told her own child to not be her true self, simply for comfort’s sake.
You have every right to be yourself, and to come out. It’s 100% unethical for a parent to ask their child to surpress their true self. In fact, it’s dangerous. Your mother has some fucking nerve. Unfortunately, her unethical point of view is so commonly accepted that she clearly has no problem at all implementing it. I’m sure she doesn’t even think about it. She even has you believing that what she’s doing is fine. You’re young, so you haven’t learned any better yet, but let me tell you: what your mother has done is absolutely wrong, it is a betrayal of trust, and it is incredibly unethical.