Board Thread:Questions and Answers/@comment-79.214.46.6-20131019043129/@comment-1739368-20131019073153

Yes, Lisa Lisa is Joseph's Mother. Yes, it was weird that Joseph was trying to peep on her. It also explained in the story that Erina kept Joseph's mother  a secret to keep him away from the danger that comes with ripple users.

Also, I wouldn't really call them step siblings either: Erina found out she was pregnant with George II soon after Jonathan died and immediately gave Lisa Lisa to Straizo, so technically speaking Straizo is her adoptive father. He has no family relation to Erina or Jonathan so Lisa Lisa is not really in the Joestar lineage, making her and George a perfectly fine match.