Not a very good title for what I'm looking for but the best I could come up with. I'm looking for a sort of story where a child is adopted or fostered by a family that already has kids, and her presence causes all sorts of drama. She's not really welcomed, at least not by most of the family, and this creates a big strain, which others might blame her for. The story should focus on a relationship with such a child. My best preference would be something like this:
a boy suddenly has an adopted/foster sister show up, and all sorts of tensions form. At first the boy resents the girl and blames her for messing up his family, but gradually he changes toward her and their bond develops over the course of the series.

As I said, that's what I think I'd like best, but much of that doesn't have to match. It doesn't absolutely have to be a girl, the person getting to know the kid doesn't have to be a boy, and doesn't even have to be in the same family. But something close to that would be great. Part of the reason I wanted it to be a boy and a girl is because I'm into love stories right now, whether we're talking about stories centered on love or stories where romance happens as a subplot to something bigger.

Also, completely fine if she's not "normal", like if she has some sort of power or there's something about her that causes bad things to happen. In fact, this would in itself be pretty cool too, since it gives a key to the other person suddenly opening up to her: he realizes that she's not really responsible for the bad things happening, she's just either being targeted by bad things or trying to warn when they're about to happen. Actually, I thought of this request after watching the anime adaptation of The Ancient Magus' Bride. In this story, the heroine does not have anyone really, she's rejected and rejected until she's bought by the ancient magus in the title. She has the ability to see fairies and other supernatural beings, and they're drawn to her power. Due to this, bad things tend to happen around her and she always reacts or moves right before they happen, leading to people connecting her to them. She wound up being taken in by relatives, and said relatives' home was split apart by conflict over her, with everyone either telling her to leave or calling other relatives to try and hand her over without worrying if she can hear. While it didn't happen, I couldn't help but wonder what it would've been like for just one person in her home to know what was going on or at least know it's not her fault and show some sympathy.

Err, the only thing that comes to mind is Sekai Oni but... there's no... happy ending with the family that adopted her. Yeah, let's say that....

Niehime to Kemono no Ou this is kinda the same as Mahou Tsukai no Yome
she was raise to be a 99th sacrifice to the terrifying demon king since she was born....

why i say it kinda same as Mahou Tsukai no Yome it's because:

- she is an unwanted child
- the "male" is the demon king (a non-human or a beast/monster)
- she learns magic or wanting to be strong because she has to be the queen if she wants to be with him (in Mahou Tsukai, Chise learns magic because she has a week body)

it's such a great romance manga