Roger's new family
While it probably might be a continuity error, or not. I might be able to explain Roger's new family, even though it's not explained in the movie, who ever has seen the first 'House', Roger had an ex-wife and a son named Jimmy. However 'House IV' is set many years later and focuses on Roger's new life.
I would assume that despite 'House's' happy ending, Roger and Sandy never got back together, and she got custody of Jimmy, so years later Roger marries Kelly who has her own kid, a daughter named Laurel.
This is the simplist explanation i could come up with.
Take it with a grain of salt.