Cerebral palsy is not so much a disorder as it is a type of syndrome caused by many different types of disorders, most with unknown causes. There are a couple of recognized genetic traits that cause CP as well as other symptoms, and there are probably some unrecognized genetic disorders that also cause CP. Without knowing the specific trait or mutation that runs in your fiancées family, its not certain how the gene might be inherited, but the pattern looks like it could be a sex-linked recessive trait.
If the trait is sex-linked it would occur in males with a probability of 50% as long as the mother does not carry the trait for CP; and with a 67% chance if the mother carried the trait, bit did not have CP.
Females could not inherit the trait unless both the mother and father carried it. Females would have a 30% chance of CP if the mother and father both carried the trait, and the mother did not actually have CP.
If this form of CP is caused by a sex-linked trait, there is a 50% chance that a son would have CP; however, since its not certain this is a sex-linked trait, its possible a son might only end up with some mild CP-type symptoms if he inherited the trait.