I agree with your sister. It's a bad idea and it might push him further away. While I don't think there's anything wrong with telling a YF you love them, I get the impression from your post that you want to go way beyond that. He probably already knows that he was special to you and how much you love him because of the way you were always there for him. But to tell a 16 year old that he changed your whole understanding of love and life is bound to sound weird to him.

I understand why you want to do this. You want the warm fuzzies you will get if he says something similar back to you. We all want those warm fuzzies, and there's nothing wrong with that. We enjoy those warm fuzzies so much we often tell everyone on the board when our YF does something that gives them to us. But it's usually something spontanious the boy says or does. It's not usually a good idea to seek them out. It might make the boy feel uncomfortable or under pressure. Personally, I think the risk of pushing him away outweigh the possible benefits.

Dakota