well this was surprisingly good. i think i love Korean movies and dramas, because it's not just face-value, but there is depth to the story.

my two cents.... U-in was pissed at his gf for persuading his father into sending him to Korea, nobody wants their partners to treat them like a child; as someone insignificant and go over their heads, it's not a good way to treat anyone in a relationship.
from the get go he was weird with his future sister-in-law. their first meet at the coffee place had a weird vibe. when u look at him. the sister was reserved but it was normal, as she was basically meeting a stranger.
maybe then he found her attractive and his subconscious chose her as a means of getting back at this gf. donno - guessing.

the older sister was leading a boring life, guess that's what routine does to a body, its depressing. the scene at the start where she is watching TV and her son is asleep with his head on her shoulder, was endearing and heartwarming. but the look on her face was "dead" and considering her son is a teen, and they usually don't like cuddling their mothers, this should've made her smile. bad indicator that the only emotion she showed was when she started interacting with U-in. he reminded her that she was alive. kind of a cliché, but loads of woman cheat / have affairs because of this reason. not saying this is ok. but this is a sad fact.

i think her ending it with U-in and then breaking up with her husband, was self flagellation / penitence cause she knows she has done wrong, so she doesn't deserve happiness or a "happy ending" that's why she broke it off... but ironically they r both on the same plane, heading to the same destination... kept open for us to make what we want with that ending...

side note didn't get why the sister broke the fish tank, they didnt f*ck ur fiancée dude.... if it was me in the heat of the moment i would be aiming at my sister - but that's me. ¯\_(ツ)_/¯ but all in all a good movie