Age wise, they would be older than the Hayden but that is about it. It would be totally up to you if you wanted her to be between Pip and Hobie or even older than Hobie. We picture them coming from a large family (Irish-Catholic) so there is even room for more siblings down the line that would give us all a lot more interaction and chances at character development.
The only real details needed (because we really want someone to feel comfortable and create their own unique character with their own unique relationship with in the family) is: Hobie is in the Army and currently stationed in Afghanistan (I'm about to bring him home). He's got a baby mama so there is a nephew floating around but he's completely and utterly a dead beat. Before leaving for his current deployment he was partying all the time and actually was not a nice guy. Those ways though are now nonexistent as he has learned the value and importance of love and life while in the desert. Little sister Pip (hayden) is 17, about to turn 18, and lives at home still. She's rebeling a bit because she's upset her brother left but is a really great kid - very loving and friendly.
Some faces we were tossing around were Kate Hudson, Amber Heard, Amanda Seyfried, Keri Russell, Drew Barrymore and Hilarie Burton. Of course really any face would work as long as they some what resemble either jt or Hayden.
There is also a cousin in the game so line wise your character would have her two siblings to talk to as well as a cousin (patrick stump pb) and then hobie's baby mama who he loves a lot. There are also a lot of possibilities too as we build and grow the family as well as interact with others in the comm as people seem really friendly there and willing to offer lines.