Talk to a local attorney about it. If the statements have been published to another person they are defamatory. The problem becomes proving damages and that will be the issue in your case. A PA attorney should be able to give you specific guidance on this issue. Good luck with it and I would just try to disengaged with the step daughter's mother. Attorneys see this sort of thing all the time and more litigation normally just gets you more litigation. However, you should speak to a lawyer about it.
The information provided herein is for informational purposes and should not be construed to establish an attorney client relationship. To establish such a relationship, the prospective client would need to meet with me in person, and have a detailed discussion about all the facts and circumstances surrounding your case.
Start by contacting the pediatrician, request that the comments be removed from the records, and request that the corrected records be sent to anyone to whom the prior records were delivered.