The main reason for Lin QingLan's hostility towards Bai LiuYao wasn't because the latter had a crush on her brother.
It wasn't even because Bai LiuYao never made an effort to disguise her feelings for Lin FenXiang. Lin QingLan's reason for hostility towards Bai LiuYao, from when they were classmates till today, was mainly because she had a great relationship with her brother who was Lin FenXiang's best friend, Bai XiNian.
It made Lin QingLan envious of her, of her closeness with her brother and the way Bai LiuYao seemed to intentionally or otherwise rub it on her face didn't help her dislike for Bai LiuYao.
Call her unreasonable, but for a girl — at that time in high school — who yearned to have what Bai LiuYao did getting the pleasant interactions and apparent care to her face was unpleasant.