as Catholics, I think what your family is scared of is the fact that you will not be granted eternal life, you will be in hell, etc. and they don't want that for you. I find that many religious people are sincere in that part. I would talk to them and don't use the word atheist. I usually tell people when they ask me is what I believe in, not what I believe in. I believe in the goodness of human beings and their power to do good whether it be with or without god. then if they have questions like heaven and hell, I just tell them I don't believe in a heaven and hell. (I am agnostic)