There are friends who can be termed as friends with benefits, they are just your friends so that they can get something from you, they are not genuine friends at all. Your friend is one of such, he is not being true to himself and your family, he is using your closeness to his advantage. Why would he want so much to know how your sister would regard him as? Is he trying to get close to her? because he could be using that question to weigh how your sister feels about him, then he jumps for the opportunity to start dating her. Like when your sister told him that she regards him as your friend meaning, he doesn't fit in as a very close friend of the family, that gives him a direct green light to go ahead with his plans, and not feel remorseful if anything happens because he knows she is my buddies sister no big deal we are just good friends. You have to talk to your friend and find out what he's really up to. I believe the player instinct is him watch over your sister.