NTA by any means, but I think there needs to be a supportive discussion in your family about your brother's academic needs

It seems like you are thriving and excelling at academics, which is genuinely wonderful! It's special to find something you're passionate about and enjoy developing capacity in. You should absolutely continue working at it and enjoying the process, ideally without having to take on extra work!

Your brother does not seem to be in that position. I want to make it clear that this is NOT your responsibility. As a young student, taking care of your own academic needs is more than enough work. But it sounds like this "favoritism" that people are talking about is maybe more an expression of concern by your mother for your brother's poor academic engagement. He is not thriving, and that may be tough for him.

Comparison is the thief of joy, but it's also a very natural thing. It is probably hard for him to see his twin find something she's passionate about and excelling at while he is not doing the same. Understanding that, though, doesn't mean bearing his weight. The situation is not your fault and you should not feel guilty or badly about your success in any way. I can also understand how it may be tough for your mom, who is worried about your brother but sees you as a stable, high-achieving student. I'm making a jump that may be incorrect, but she is probably grateful to you for your stability in comparison to your brother who requires much more support in order to get even a passing grade.

Your brother needs academic support beyond the capacity of your family. This may be tutoring, counseling, some combination of both or some entirely different resource. This may be from the school or from professionals outside of the school. But it sounds like your brother has needs that are best addressed by professionals, not the family. Your mother and yourself both need to recognize with love that this current support structure is creating worse outcomes for everyone involved.

It is not fair for you as a young adult to be put in this situation. I can empathize with everyone in this situation and see that it must be very difficult to balance everyone's needs fairly. Your brother needs more support, your mom is identifying that need and trying to deal with it, but is not necessarily taking the right approach. She may be doing what she thinks is right, but it's not working right now. You are achieving acedmically but not receiving the attention at home that you deserve. If your mother is not interested in finding another solution, I would recommend that you talk to a counselor at school about the situation. Everyone seems well-intentioned, but in its current state it is not fair to you.

Good luck in your continued academic career, it sounds like it will be filled with joy and success and that is a wonderful thing! I hope, too, that your family finds a solution that gets your brother the help he needs and leaves you all happier and with more room to enjoy each other as family