judging from all the comments ive read... is that hajin IS the first born son of his father out of wedlock while his younger brother is his father and his wife's biological son... hmm that explains the wife's actions towards hajin then ig.
good thing it was just a side story to show no matter what situation they're in beom will always find a way because thank god his grandma is still alive in the main story
kim dokja cale henituse ahh skill