How can you hook up with your sister's guy friend if you went out twice with others but you don't know if he likes you and you are both really shy?
Tell your sister how you feel and perhaps she can come up with a
plan for the 3 of you to go out together (just to begin with.) It
would be a little more personal just the 3 of you and you'd get to
know each other more so one of you won't be too shy to ask the
other out. Good luck!