Nightmares belong to the infraconsciousness, so if your family members want to overcome those types of hypnotic, terrifying dreams, they must convert the infraconsciousness to consciousness.
There is no better way.
In the meantime, if they are accepting of the method, they can pray to divinity to grant them peace during their sleep. Prayer would be particular to the religion they may be a part of.
If this is not the case, then it can get difficult in trying to help atheistic people try spiritual techniques.
However, the Pater Noster, the Ave Maria, the Fatihah of the Qur'an, the Conjurations and Invocation of Solomon work wonders, but there must be faith involved in the recitation.
For thirty years I sought God. But when I looked carefully I found that in reality God was the seeker and I the sought. -Bayazid al-Bastami