The Skywalkers (Star Wars): The Skywalkers are more or less the gold standard for family dysfunction. Putting aside that twins Luke and Leia have the hots for one another, Papa Vader chopped off Luke's hand, nearly killed him, and annihilated Leia's entire home planet. Makes those family dinners pretty awkward.
The Cylons (Battlestar Galactica): The Significant Seven models of Cylon are a big, dysfunctional family in their own right, albeit one that pulls out the heavy artillery when a disagreement over the family pets goes to far. But it's within the individual model lines that the dynamics get particularly screwy. When everyone shares the same face and the same programming, personal identities and relationships tend to get blurred. At least one of the Sharons wants to be Sharon Agathon so badly that she sampled her fellow Eight's memories and put the moves on her husband. And then there's this scene: