Saying that to the sister was… shocking, but the bitch deserved something to hurt her, and it seems like nothing was going to hurt more than knowing that (I also believe in the theory that she has a crush on Doyeon). Jinju's character is certainly intriguing, when I think I start to understand her, she does something that confuses me again, and for that I love it.
Doyeon is not a complete saint either, I don't see any red flags in her yet, but staying fucking her neighbor instead of going with her sister or talking, maybe it wasn't the best alternative XD