It was a short shojo manga, but I've forgotten the title. The protagonist's father is a stepfather. Although they share no blood relation, she believes she was raised with great care. That is because her biological mother was a terrible woman who neglected her children and home, constantly running around and partying. The stepfather handled everything that a parent should do. The stepfather was always submissive to her short-tempered mother. As she grew up, the protagonist often wondered, "Why does he stay with a woman like that?" She had never once heard him speak ill of the mother, and whenever the protagonist brought it up, she would be gently reprimanded. "Your mother has a hard time, too. Let's be good and stay home together, just the two of us." While holding onto these doubts, she learns on TV about the notion that couples sometimes stay together solely for the sake of their children. While all this is going on, she happens to witness her mother cheating one day. Furthermore, she hears her mother say, "I only married the stepfather for his money." Back when the affair was exposed, her mother had been dumped by the protagonist's biological father, and the stepfather was the man she found when she was looking for a convenient tool. "He had zero charm as a man, but he was completely devoted to me and did whatever I wanted, so I made him my husband."
Having finally cut off any remaining attachment she had to her mother, the protagonist tells the stepfather everything and suggests a divorce, but she still doesn't get a positive response. That's when it suddenly dawns on her. Could it be that he isn't getting a divorce for my sake? He might think that since we aren't blood-related, we would be separated. So she says to him, "Are you worried about custody? Of course I'll choose you, Dad. As long as I have you, I don't need anything else." The words the stepfather finally spoke were unexpected. "Don't make any mistakes. Everything I've done raising you up until now was entirely for your mother's sake. I kept at it hoping that one day, she might come to love me. There's no parent in the world who would want to take care of a child they share no blood with, to the point of getting a divorce over it."