Brother tries to bring back his non-blood-related sister with him so that he can support her, but she does not want to be a burden on him so she repeatedly goes back to the uncle's house who is a drunkard (the aunt is a good person though) drowned in debt for his drinking habits and tries to sell the sister to clear his debts.
This is my impression from the scanlated chapters (3, in English).
The story could make for a very intriguing one where siblings defy odds and surpass all the barriers that people around them pose, but no, the author had to make it into
some cliche story. Mind me, there's nothing wrong with being cliche. The problem lies within that it tries to follow a bad cliche to attract attention of viewers but falls flat on its back when the epilogue to every major arc is brought up.
I would have liked it if the sister wasn't so defying of her brother and tried to support him at least. I feel bad for the brother who worked his ass off to save so much only to have his sister disrespect his efforts. The storyline makes the brother only a muscle-brained maniac that will fight for his sister and a sister that refuses to acknowledge her brother's efforts. Plus the subtle non-blood-related sibling love trope, which is very poorly incorporated into the story and doesn't blend well with the overall premise.