Oxcarbazepine,the drug your wife is taking,effectively at that,is known to be harmful to the developing foetus,though no long term studies are available in humans.
You really have to balance between the any drug's effectiveness and it's side effects.
It obviously is very effective for control of your wife's seizures,so can't be stopped.
Their is always a risk to the foetus,but there being no long term studies in humans,it is a difficult decision,but one to be taken ONLY by you and your wife.
Next Steps
consult your gynaecologist too before deciding.
best of luck.