Does anyone have advice regarding epilepsy?

My sister in law is at her wits end with this. I feel so so awful for her.

She has two children. 6 & 2 years old. They both have epilepsy, although different types.

They are desperate to have another child but they are scared that baby will get epilepsy as well.

Nobody in both sides of the family have epilepsy. Herself and husband (my partners brother) think their DNA isn't a good mix. They truly think they make poorly babies because their dna doesn't match and causes this.

I mean, what are the chances of two children have epilepsy!! The hospital doesn't understand why either.

My two children do not have epilepsy, it doesn't run in the family.

Could this be true? Their dna don't match??