I think that with how close Jack and Rusty are, it would make perfect sense for them to play together at each other's houses. Because of this, Lulu would definitely know Rusty... and maybe she'd have a crush on him.

  I mean, Rusty's a really nice guy. Big (from her perspective), strong, loyal, brave, manly, understanding towards her ADHD brother... what's not to like?

  Jack might initially be confused, but he'd come to accept it.