There are a number of creepy aspects to Harry and Sirius' relationship. They glommed on to each other far too quickly at the end of PoA, all ready to go off and live together and play Happy Families, when neither one actually knew the other as a person. It's not that their love for each other isn't genuine, it's just that it's so muddied by external issues. I think one of the most tragic aspects of Sirius' death is the loss of their chance to properly get to know each other and develop a healthy relationship.