My son was having 3-4 night terrors a night, every night. I went to a couple of different peds. and did research on-line. Here is the information and advice I have from my experience.
*Make sure he is getting enough sleep. Try a bedtime 15-30 min earlier. Make sure he gets a good nap. Fatigue is the most common cause of night terrors.
*Look for a pattern in what time they are happening. If there is indeed a pattern, your sister can go in 10-15min before the night terror is to happen and change your nephew's sleep pattern. DO NOT WAKE HIM! Rub his back or roll him over. Stir him enough that it disrupts his sleep but not enough that it wakes him up. This changes the sleep pattern. Do this for 3 or 4 nights in a row. Hopefully after that your nephew's sleep pattern will be changed and free from the night terrors. My pediatrictian said sleep patterns typically change every 12 weeks on their own. My son's first phase of night terrors lasted almost every night for the full 12 weeks. The only nights he didn't have night terrors were the nights I went in and rubbed his back before they happened.
*You could also have his iron level checked. My son had night terrors from 14mos-3yrs. My ped. said there is new research linking low iron levels with night terrors. My son's iron levels were borderline. He took a prescription iron supplement for a short time. After that his night terrors stopped. It almost was too good to be true! He still has one every now and then. Most kids grow out of them by age 6. However, some adults still have night terrors.
*For some kids a night time bath can be stimulating. Try changing bath time to the morning instead of in the evening.
*A cooler room is better than a warm room. Make sure he isn't over dressed at night.
I wish your sister the best. Night terrors can be so scary. Let her know that the child actually has no recollection of the night terrors happening. It is scary for the person who is observing the night terror.
God bless!