Greek Irene:
Midnight_Blue raises a good point on Inn's feelings on the whole matter. Which makes me think that Inn might become an antagonist later on as she's trying to "get back" at Irin.
I don't think Inn would necessarily become an antagonist because we have to think about Jin's role in this as well and also their parent's and the role, they play in it as well. I feel like how Inn feels is bigger than her sister's treatment of her but more so everyone's treatment of her with the exception of Jin, since it looks like he has something for her by the look he gave her about going to the hospital and her sister's whereabouts. Furthermore, this isn't a matter of her "getting back" at her sister because of the way the situation is outlined. When it comes to Inn's feels for Namcha, she isn't blinded by her feelings as she knows that he only loves her sister, so she manipulates the situation of Irin getting married so that she could get what she wants, which is his attention - good or bad. Which Irin plays into this because again, she knows that her grandfather wouldn't allow her to marry Namcha who she loves due to his age, so she wouldn't dare to stand up to him but only accept. So, this makes her extremely jealous when Namcha shows attention to someone else even if it's her own sister of all people.
I feel like Inn has animosity towards her sister if anything due to all the degrading that she has done to her and her constantly overshadowing her. What I really love about this show already is that for two episodes to be already out theirs a lot to unpack for two episodes already. Even though, I know how her character is going to unfold since her intentions were clear since episode 1. I do understand how she feels, which makes her one of the most interesting characters of the show.
Unpopular opinion: I don't care for Irin because she plays into the whole idea that because one is young that makes them Immature, inexperience, and childish. Which is something that both Namcha and Inn are trying to escape from specifically Namcha as his mother gave him important advice.