I like the sister. Not because she's more beautiful, but because she has her shit together. Having low confidence, not working on it, wallowing on self pity, and becoming reliant on others to give you some sort of self worth is not healthy. That's why the MC felt unworthy and ugly when things didn't work out with the ex. She projected those feelings on her sister and her sister was like "no bitch, I know I'll find someone else". There's different things you can base your confidence on. All the MC focused on was on something subjective like beauty. She's like a racing horse wearing blinkers. Too caught up in not being as pretty as her sister (whose career is based on physical appearance lol) to see the big picture.
Don't have any confident and self pity are two different thing you know. In this case, she's self pitying herself. I encountered the novel first and didn't even managed to finish the first half of the novel because I'm annoyed with the FL. In the novel, she even have a coworker who wanted to court her but she feign ignorance because she didn't interested. The next thing, she had a gal to think that everyone attracted to her sister and she's not beautiful enough to be courted. Urghhh
Sorry. I'm ranting
>:Z idk guys... I hated the FL