Is it possible she resents the attention you get from your mom?

(Btw, there's definitely a big age gap between you so it's a bit alarming she'd be physically abusive towards you?)

I say this because it seems like she wants your mom to buy her the things she just bought you and has been abusive towards you.

I'm also wondering if there's a bit of resentment from you that she copied a goal you had for yourself and had some success in it. She sold her books so it sounds like success for her.

I also wonder.. twins generate A LOT of attention.. then 14 years later, you were born.. taking away their spotlight at an age when she was likely figuring things out for herself. Maybe she felt emotionally abandoned?

Talk with her. Either she admires you or is competing for the attention you generate. That's my thought, anyway.