Canine Epilepsy occurrence among litter-mates

So, my dog's sister was just diagnosed with epilepsy. :S

They are from a litter of 10 puppies - German shorthaired pointer / lab - 9 girls, 1 boy. Both parents were purebred.... but Dad jumped the fence.

They are 3.5 years old.

My dog is showing no symptoms.

Does anyone know what the % of epilepsy among litter-mates is? Also, are there any early warning signs I should be looking out for?