This is probably terrible advice! But, I was researching possible birth defects caused by taking Lexapro during pregnancy and I came across this: Craniosynostosis is a congenital (present at birth) defect that causes one or more sutures on a baby's head to close earlier than normal. Sutures are connections that separate each individual skull bones. The early closing of a suture leads to an abnormally shaped head.
I have no idea beyond that paragraph right there anything about it or anything else! The best advice for your sister is to check with her own Dr. If she is really worried about it have her make a special appointment and have the Dr. look at it and talk to her about it, don't wait until the baby's next well visit. No reason to be worried when all you haev to do is have the Dr. look at it, I say.