Ah, I see. Because everyone is gone, Satou feels entitled to her parental role to the point she couldn't allow herself to be selfish.
Though, I can sense there's probably more about the sisters. There's probably a reason why her sister is #1 top priority, perhaps something bad will happen if her sister is left alone?? (self-harming for example) And after her sister is all good and stable, Satou still couldn't leave her.
Or maybe she thinks that if she were to find love, she'll leave her sister all alone like how her family left her. These are my guesses anyways, looking forward to next chapters!