I suspect you meant to say your sister is on 250mg of Eptoin
? instead of 25mg as mentioned above. (as 25mg is too low a dose and unlikely to provide any meaningful protection against seizures)
The risk of seizures is highest in those with multiple lesions in the brain especially those with surrounding inflammation.Even in people with calcified inactive lesions longterm antiepileptic medications is necessary if the patient has recurrent seizures. In some it is possible to taper and stop medications after to 6 to 12 months of radiographic resolution of active infection.
There should be no problem in continuing the antiepileptic medication when your sister gets pregnant.If she is apprehensive she can be switched to newer drugs like Levetiracetam
which should provide the same quality of protection.She could also discuss with her Neurologist
regarding a trial stopping of medication.
Hope this helps!