Britney Spears' sister JAMIE LYNN was left heartbroken after finding out the father of her unborn baby had cheated on her. The 16-year-old confirmed she is pregnant with student Casey Aldridge's child earlier this month (Dec07) but it is now alleged he had been dating other women behind the Zoey 101 star's back. Aldridge is accused of being romantically involved with Whitney Seals from Magnolia, Mississippi, around the time the actress announced in an interview with OK! magazine she was to be a mother. Seals' brother, Kevin, tells MSNBC.com, "Whitney and Casey have been dating for months. I know Jamie Lynn was really upset and jealous when she found out about it."