I’ve been thinking since the beginning
I'd argue its more of a control thing. Like because he suddenly has to share space with another, while technically being the less capable 'twin', his anger and bitterness has made him want to at least have the upper hand. It's knowing that Seo-In fancies another while hating him that makes Jaewon demand a kiss, to prove that he can still force Seo-In to do things he hates. There's a pretty strong correlation between sexual acts and having that sense of control/domination that's separate from romantic interest. We see this in other media too like Banana Fish.
I know right I absolutely agree while in season 1 when the backstory hasn’t came out , I automatically assumed Jaewon has a crush on Seo-in but in season 2 after reading the backstory I absolutely believe that it’s 100% a control thing
Does it seem like the brother was in love w him