After opening Bart's present, Homer said that opening another would be like following the London Philharmonic with Jessica Simpson. In the Springfield Grocery Store, Lisa picks up a copy of Peephole magazine to see what "cousin Jessica" is up to.
The reference to Jessica Simpson is a joking nod to the real-life singer that shares their surname. However, Lisa may have not actually meant that Jessica really is their cousin. In actuality, the real Jessica Simpson has expressed interest in an appearance on the show where she is related to Homer. Abe could be her paternal grandfather. Herbert doesn't have a kid, but it is possible that Abbie does, meaning Jessica’s maternal grandfather is unknown. So it possible if she is one of Abe's brothers's daughter or Homer's cousin Frank's daughter or she could be Homer's younger sister, so she may be Abe's half sister or Marge's half sister.