I've got a feeling that the sister have some brother complex that's why she doesn't give him the medicine so he can continue being the last omega, without an alpha, he will just stay by her side. That's why when she saw him talking with another guy, she suddenly goes out of the car to go to his brother.
When he heard about Doyun liking him for a long time, he suddenly in rut. Hahaha.
His heart skipped a bit. (づ￣ ³￣)づ
It's not that messed up. It's actually pretty cute to me.