false religions

I keep reading in the Bible that if someone believes in a false god or teachings, you should not eat with, let into your home, or associate with them unless you are witnessing to them. I have mormon family members and not sure if I can have them over, go visit them, or be around them. It has been suggested to me that since they are family, the rules are different but I can't find it anywhere in the Bible. Can you give me any insight?