I always thought these celebrities could be parents and kids:

Phoebe Philo and Daria Werbowy

Daria even got the Phoebe makeunder as the new face of the Celine 2013 campaign

Tommy Lee Jones and Josh Hartnett
Kelly Rutherford and Whitney Port

Whitney would have made a good Serena on Gossip Girl The celebrities below would make good siblings:
Lauren Conrad and Carmen Electra
Katie Perry and Zoe Deschanel
Keira Knightly and Natalie Portman
Minka Kelly and Leighton Meester

Chelsea Handler and Elizabeth Banks