This is an AU where Narcissa is sick of all the pureblood supremacy so she takes Draco and leaves. She doesn't know what to do, so she goes to Andromeda for help. Dumbledore organises a trial for Sirius and he is proven innocent, so he gets custody of Harry. He doesn't know how to raise a child so he goes to Andromeda for help too. In this story Nymphadora is the same age as Draco and Harry. Three Blacks come together to avoid the pureblood values of their family, maybe one day joined by a fourth.