I get where ur getting at but yoojin only liked seryeon after meeting him at convenience store which was after yoona started to date seryeon since they dated for 6 months(?) the sister ended up breaking up with seryeon once she picked up that her brother was catching feelings for her bf.
And I don’t think the sister only dated the actor bc it was fun I think she did have feelings for him but it most probably didn’t last long and she wasn’t able to break up bc she felt bad for him. From what I picked up it seemed like she was trying to get out of the relationship when she mentioned that she was friends with her ex and that they’re close she must have thought that bringing that up might push her bf to leave her and this could also be why she never addressed yoojin as her brother when she brought him to meet seryeon so that her bf would think it’s her ex and break up. Which is totally immature and irresponsible bc ur playing with someones feelings.
But ya i agree with the boundaries thing it’s totally not cool.
am i the only one who really got irritated with his sister??? she knew he likes seryeon yet she dated him? and for what? for fun???? and then she uses yoojin's phone without consent, yes they're siblings but she's going over his boundaries. she used his phone one time, now it caused a ruckus. omg im so irritated. seryeon nd yoojin was fated FROM the START. she made them knew it each other but im still so irritated with her.