I read your query and I understand your concern.
As much as us doctors would want to have the answer to that question for our patients unfortunately in a patient with epileptic seizures it is not possible to predict with certainty whether the patient will respond to treatment and whether the seizures can permanently disappear.
There are patients with epilepsy who don't respond to treatment at all, there are those who have a reduction in frequency but still have seizures and there are those who are free of seizures under treatment like your sisters case.
In this last group if they are seizure free for at least 2 years we usually try to lower the dose gradually until interruption, sometimes they remain free of seizures but in others like in this case seizures reappear. There is no certainty on who will have seizures again and who won't. Since in her case they reappeared, I fear that there is the possibility of her needing lifelong treatment, you could try again in 3 years as your doctor said, but should be prepared for the fact that that might not be possible. An EEG before trying would be an indicator (but still no guaranteed results). Also I wouldn't recommend trying over and over again in time, because it's a question of a delicate chemical balance which could be disrupted and there are studies which have shown that in a (small) percentage of cases if an effective treatment is interrupted and then had to be restarted this time seizures could become resistant to treatment.
I forgot to mention that it also depends on her age and type of seizures. There are some childhood epileptic seizure types which have a good chance of disappearing when she grows up, but you don't say her age. If she's already an adult then what I already said applies.
I hope to have been of some help.