Reminder MC got offered to tiger girl when he was a baby, then after growing up got forced to associate with her. At the end he did decide to stay with her, but both staying together is a decision both made. Little sister doesn't care about MC, but she sure is getting so upset over the choice they made. She brings points why the union is difficult to say the least, but that are theirs to overcome. Feels like she's a parent trying to intervene in her adult kid's couple affairs (adult as in she can't just forcefully break the relationship). She's only seeing from her side, with her sibling's well being/future, and zero consideration to Sung Hoon's circumstances (love lived/plenty knowledge phantom vs young and still learning human).
Plus, I feel she's being overprotective over Rangii to the point of stifling her growth. She's going nuclear over something pretty "banal" as couple. True, a toxic person can destroy another person, but Rangii's over competent servant is not going to let that kind of person approach her in the first place, meaning Sung Hoon is worthy. Rangii is never going to grow if she doesn't experience anything, both good and bad things in life. No wonder little sister isn't at Rangii's side, she must be stifling to be around 🤣