Very well constructed. Most would've taken the easiest route and assume abuse. I honestly love the part where she censored the flashback of her brother, because ultimately, it was from her perspective. It could be abuse, it could be pity, it could be something entirely different. But whatever it is, it is traumatic to her.
It's true that she has a very active imagination. It's true that she's smart. It's also true that she has a very low self-esteem. A combination of all these is not pretty, and her mental state is definitely compromised (I would know). Add to all these is the negligent parents who's biased towards only one child (not really that uncommon sadly).
It is possible that she has been abuse mentally or physically or both by his brother but what if it just her overthinking. I still fully believe their parents is not a good parent and favour her brother. However, her brother doesn't seem to be petty but more like clueless. She can't remember even single thing bad her brother do but only remember what she see of him. She is someone who give hasty judgement of people, dont forget that. She assume a lot of stuff as well and overcomplicate stuff. She might be insecure and plus her parent reminding her that she is not good enough make her even more insecure. Her brother probably try to be close to her or thought she probably just like that. Awkward. Also she is talented but dumb about it. Maybe it just in family to be humble or dumb about their own achievement.