A
The bible even says some have left the truth (that Jesus is God). My cousin did. She left Christianity to become Muslim. She knew the truth her whole life, into adulthood, but gave it up in order to marry the one she wanted to marry. She chose that husband over God. It can happen, I've seen it with my own eyes. And the bible says if they leave the truth they can never come back. How can you leave some place where you have never been?
Will pray with you for your cousin and her family.