I think it doesn't get explained properly or showed. If memory serves, after a few episodes it is told that when Min Tae Seok comes to tell Hae Gang's mother that she is dead and give her her daughter's things, somehow they talk and they decide for his brother to live with her for a while or something. I guess his brother doesn't like his family ect, he likes peace and people being silent. Hae Gang's mother wasn't able to talk due to the shock and he liked that, plus i guess she wanted someone to take care of, so everyone is happy. Something like that i guess.