
Where was his sister when he was living as a fucking drunk? Maybe I missed it but I don't remember her being around? So if I'm wrong drop the chapter...
If not... Why does she care all of a sudden? Someone correct me if please

It's not impossible that she suddenly learnt that the teacher (that bastard who looked down on Nakyum till the beginning !) was promoted and realized Nakyum wasn't with him or something like that. So she starts to search for him or she could have learnt he was in the house of Seungho who is known for his dissolute life and really worried about Nakyum.
I don't want to blame the sister for not taking care of Nakyum, she may not known waht happened to him till now.
This makes me happy.