As terrible as I think Yeonju is behaving, I get where Hari is coming from. You want to be there to support your best and closest friend even when they’re acting crazy. It’s the difference between a true friend and a fair weather friend. So far, Hari is framed as the type to stick with people she’s close to even through the bad times, that’s a strength and not a weakness. She’s also shown to be selective about who she gets close to. She felt something was off about Yuha, that he was faking a smile and wasn’t truthful with her so she decided to end their dating.
Again, we’re also starting the series off when the friendship is in a bad spot, and we never got to experience the good moments. So it’s easy for us to want Hari to cut off Yeonju. That could be a drawback in writing terms Because we as the audience dislike Yeonju and don’t feel the pity and sadness Hari is probably feeling. But by starting here, we’re at the point in the story where things are most exciting/dramatic.
I don’t agree with her fake dating Yuha and instead a dose of tough love is what Yeonju should get. People can be in denial about their partner’s faults even when it’s staring them in the eye. Think of all the times in real life we wonder why a friend is staying with a guy even though we both know he’s awful, I.e. cheater, beats her, steals, or any other major flaw.
I actually prefer Yuha’s reaction to a lot of characters in Manga/Manhwa etc when it comes to step siblings. It’s weird for characters who grew up from childhood as siblings to be attracted to one another. Here they became step siblings when they were much older and Yuha already liked her and wanted to date her before she became his sister. So now he’s trying to hold back whatever he’s felt so as not to break up his new family.
Sorry for the essay, I just wanted to be clear on my own subjective thought process 😅