Where do people get the idea that Teddy Lupin had been raised by Harry and Ginny? There's no indication that Teddy ever lived with them - James offers to give up his room, as opposed to, "Teddy can have his own room back." They're close with Ted, as they would be because Harry is his godfather. And since Teddy's left school ( I assume, given he's nineteen), he's been eating at the Potters' fairly often - and with his girlfriend at school, he'd be rather at loose ends, I suspect.
Teddy has a grandmother, one who is probably as formidable in her way as Augusta Longbottom. Why wouldn't Andromeda Tonks raise her own grandson? Why would she allow a pair of teenagers to take care of him? It makes no sense.