That's why I dont like this kinda plot for a romcom.
If the relationship didn't work in the past are we supposed to believe suddenly everything will go absolutely right... Even if now they are step-siblings? It'd not like they are more mature people, they literally broke up some weeks ago.
Specially feels forced knowing apparently MC just got jealous of her being close to other people (those people didn't even appear to be other guys so why?) and she just automatically decides to kinda go hostile as well instead of just accepting the apology but telling him she doesnt want him to act like that again..
Edit: based on her comments on the next chapter, it seems the girl was also jealous of MC being a bit more extroverted and apparently popular with some girls, but simply was too shy to voice it out until MC voiced his jealousness first.