Martha Wainwright: Laurel & Hardy
Rufus Wainwright: April Fools
Loudon Wainwright III and Kate McGarrigle were great folk singers in their own right. Sadly, Kate died last year, but Loudon still is going strong. The marriage between Loudon and Kate didn't last but the union left a lasting legacy nonetheless. Their children, Rufus and Martha, have become talented musicians on their own. Songwriting is obviously in their blood.
Martha's song "Laurel & Hardy" from her 1999 EP is about her brother Rufus and makes it clear that they have a close relationship, something that was probably fostered by their bitterness over their father not being around for most of their childhoods. Rufus is the more well-known of the two (at least so far), though, and has found much critical success.
After Loudon and Kate's divorce, Loudon had another child with another folk singer, Suzzy Roche, of The Roches. The child, a girl named Lucy, also has become a musician...as a bonus, here is a track of hers.
Lucy Wainwright-Roche: Chicago