They were half sibling. He's son of her dad and the maid. And I think the maid had to leave the house after giving birth, as they show us she was telling the little girl she has to take care of the baby if she -"the mother (maid)"- not here by their side. And when the accident happened, the mother -"the wife"- covered it and didn't save the baby.
https://www1.ondramanice.tv/drama/senior-detail
https://www1.ondramanice.tv/drama/montage-detail
And when the accident happened, the mother -"the wife"- covered it and didn't save the baby.