Why is it so rare for romance stories to give her a little sister he likes? Not like that. Not a romantic rival, but a package deal he accepts.
She gets all the tragedy and responsibility of motherhood forced upon her too soon without the baggage of teen pregnancy or being a single parent with an unwanted child from a failed relationship. And he gets to be a good person who takes care of someone else's child, without any familial bond or emotional obligation directly tying him to her.
He chooses to be the stepfather who stepped up. He gets to show his woman what a mature and loving and reliable big brother and father figure he can be. He can play the big brother and father role and get better at both.
And it lets the little sister be old enough to be her own person, her own interesting character with her own arc, not just a helpless object like a baby who needs to be taken care of constantly or she dies. The little sister can test the man in ways the big sister would look disgustingly manipulative for even thinking of. She can try to "protect" her "naive" big sister from heartbreak by sabotaging him while maintaining plausible deniability. The little sister can directly try to sabotage the relationship, or try to fix it in stupid ways, and if she fails, she'll be forgiven because she's only a child who didn't know any better. The little sister can create problems the big sister would look naive, childish, deranged, stupid, or even evil for causing herself. She can get away with things any friend or parent or roommate would be kicked out over.
It even gives a happy loving couple an excuse to not bone right away. They don't want to do or say anything age-inappropriate around her, even if she wants them to. Especially if she wants them to.
It even comes with scenes that write themselves, like the little sister eventually growing up and moving out on her own, emptying the nest so he can fill it.
It's literally perfect. In a genre so stagnant and desperate for cheap gimmicks and emotional manipulation and artificial conflicts that exist solely to pad runtime, why is this so rare?