Can anyone tell me why one child from an epileptic mother can have epilepsy and the other doesn't?

Epilepsy in Families: There are many causes of epilepsy. Sometimes epilepsy runs strong in a family, sometimes it appears not to run in a family at all. There are a few cases where you might expect about 50% of kids of a mother with epilepsy to inherit the condition, most cases though there would be a risk of 25% or less. It depends on the cause of the epilepsy. There are dozens of different causes.

Not dominant trait.: While children of parents with epilepsy are at higher risk for developing the disorder, epilepsy is not a dominant trait that is handed down to all children. Epilepsy can be caused by many different factors, not necessarily a genetic mutation.