A recent article reveals that a brother-sister half-sibling pair fell in love and continued having children even after they discovered they were closely related. One of their four children has severe epilepsy.
It seems that many different genetic mutations can lead to epilepsy, but I wonder if most such mutations are dominant or if most of them are recessive (requiring inheriting the mutation from both parents for epilepsy to develop). I suspect that these two half-siblings probably have a recessive gene causing epilepsy and that is the reason their child has such a severe form of epilepsy.