Her aunt? I mean the woman she lives with at the beginning, isn't that her mother?
Others have already explained, but I wanted to stress that PaleGrass gives the players the ability to define the relationship as they see fit. If you want her to be Lisa's mother, she can be.
However, if you want the relationships to be true to what PaleGrass intends them to be, then Lisa's mother and father are dead, and Sharon (the woman who raised her and with whom she lives) is her mother's best friend, who took Lisa in after her parents died.
So, if you want the official relationships, Lisa and Sharon are not blood related, but Lisa considers Sharon her aunt. This also meant that Lisa and Paul are not blood related, though she considered him her cousin, since he's Sharon's nephew. Of course, Paul is no longer in the game, so that doesn't matter.