My sister in law has done this. She has her own, and she took in a relative who's family had passed.
She loves them both as much as any parent could, and I've never heard her refer to the relative as anything but her daughter.
Come to think of it, I have a friend who had 3 of her own and took in her sisters daughter.
Anyway, yes, I've seen it with my own eyes. These children feel loved, and you wouldn't know any different that they started out with another family.
If that child comes to you, your heart will make room. Good luck.