Back to the Future III had a similar problem. Writer/producer Bob Gale and writer/director Robert Zemeckis were once asked to explain how the same actress ended up in two branches of Marty's family tree:
Q: Lea Thompson plays Maggie McFly, Marty's great-great-grandmother, as well as Lorraine, Marty's mom. But Lorraine's family name is "Baines". Why did Lea play Marty's paternal great-great-grandmother, when she's really not part of that family? Is there something kinky going on in the history of the McFly family?
A: Lea plays Maggie because we didn't want to make a Back to the Future Part III without having Lea in it, especially in a "Mom is that you" scene! Of course, we thought about whether it made any sense -- obviously, Maggie McFly and Lorraine Baines cannot be blood relatives. But we did come up with a satisfactory answer: It's a well known adage that "men are attracted to women who remind them of their mothers." Clearly then, when Seamus married Maggie, that insured that the McFly men would have a genetic trait that attracted them to women who bear a resemblance to Maggie or Lea Thompson (even Jennifer is the same physical type!) [Link]