My mum had epilepsy, had 3 naturally healthy child then had my sister who has Spina Bifida. During all pregnancies she had standard dose of folic acid. None of her or my dad's family have any family history of neural tube defects. She was told my sister's SB could be the result of epilepsy medication interfering with folic acid. But I have also read that babies can be born with SB without any family history of it. Im confused what are my chances of having a baby with SB is? 
Any advice?