I think the girl and mc are sibs ╮(￣▽￣)╭

woahh thats a good observation but has the mc mentioned siblings at all?

He didn't, but there is a possibility of his mom having another child and mc isn't aware since the mom is out of his life from when he was a baby

I doubt it. MC would have been 18 and Minjeong is probs 22. So she’s older than MC, MC’s dad would have known she was pregnant before.