I don't really have too many details for the sister. I see her as the youngest (that is totally up to you, though), and Michael as the oldest of three children. He definitely has that protective older brother vibe - he was a SEAL sniper, and is now a cop - and some habits are very hard to break. I've left most of their family background open (other than the fact that they are from Virginia beach, their father was in the Navy, and their mother was a nurse.. I also have their grandparents as coming from old money, so they had that spoiled rotten stage in their lives).
As for the best friend, I can go anywhere from childhood best friend, to someone he met later on when he moved to California after joining the Navy. They have a very close relationship, and she's both put up and called him out on his shit (he's got plenty of that) for a long time. She would also be the one who talked him into moving to NY after he left the Navy and was dealing with his PTSD.