There was a flash and suddenly Sarah was gone. In her place Janet was stunned to see an identical copy of herself, wearing Sarah's now much too tight clothes and looking more then a little confused. Janet couldn't believe it. She had messed up again. Rather then making Sarah her daughter again she had made Sarah into a copy of herself. For the stone interpreted her wish by making Sarah a genetic copy of herself.
This time Sarah freaked. Being changed suddenly into a woman twice your own age can be traumatizing after all. She grabbed the stone from Janet. She wasn't sure what was going on, but it was clear the stone Janet held was the cause of it so she figured she had better get it away from her. As far as she knew she was Janet's stepdaughter and wanted to get back to the way things had been.
In hindsight she could have phrased her next wish better though.
