Magic Changes Relationships

Both Lily Evans and Sirius Black lost a sibling when they accepted their place at Hogwarts. Petunia begun to get jealous and isolate herself from Lily. Whereas, when Sirius got sorted into Gryffindor, his brother Regulus and the whole family isolated him didn't talk to him at all. Both found solace in each other, but knew that a friend is not a replacement for a sibling.