Edgar might do some ~older brother scolding when it comes to Sirius, but he's just a concerned kinda guy. All in all, he'll be impressed with the things Sirius gets away with, and live vicariously through him. They'll be friends.
Cassandra can count on Edgar to be sweet to her, too. In a subtle, helpful way.
As for the rest? Rodolphus will probably bring about Edgar's end someday, et cetera.