Yeah i quite agree(and hope it will turn out) that its either the sister "limit" herself, misunderstanding from both of them or the combination of it.
Because i think this shown by the fact:
1. The sister quite hardworker and have big motivation recently
2. The sister still indeed talk to MC, and from the talk its actually not that bad, its ositive actually,
A. She still ask his brother
B. She tell his brother to communicate more
If she really hate his brother, she should stop talking to him totally or call him insect or something
I really hope its just misunderstand ing