My Sister's "Friend"

My sister Carli and I are only 13 months apart so she isn't that much younger than me so I basically know everything about her. Well she is keeping something from me. This boy showed up at our house last night but I don't know who he is...he came over today too and swam in our pool. They talk on the phone a lot too and she hides everytime they are on the phone. Which is soo weird because she usually lets me listen when she is on the phone just so I know she's not getting into any trouble. But she is giving me no information but this guy!! He looks like a nice kid and my parents like him and he is cute...but are they friends or more than friends???? Heck as far as I know he could be my long lost brother! Just kidding. But I really need to find out whats going on with Carli and this kid!!