My best guess from reading the 8 chapters + comments: ML and his sick sister were the children of the wife. And instead of loving them like he did the FL and her mother, their father abused the ML and his sister. Hence why he asked if her father used to beat her and he said he was jealous because she experienced something he never would. Unless the author is twisted enough to truly make it incest, the ML is probably not the father’s biological son.
My best guess from reading the 8 chapters + comments: ML and his sick sister were the children of the wife. And instead of loving them like he did the FL and her mother, their father abused the ML and his sister. Hence why he asked if her father used to beat her and he said he was jealous because she experienced something he never would. Unless the author is twisted enough to truly make it incest, the ML is probably not the father’s biological son.