Rett syndrome is genetic, but generally not inherited. It is most commonly caused by a sporadic mutation on the MECP2 gene and can happen in any family at any time. There are several multiple cases of Rett syndrome in families worldwide, but these are very rare. Below are some useful resources explaining the genetic basis of Rett syndrome.
ABC of Rett Syndrome A General Introduction
Genetics – Questions Parents Ask
Rett Syndrome A Useful Summary