I want to come out to my parents that I'm bi?

But they would tell my grandpa, who believes that anybody who isn't straight is completely immoral and will burn in hell. He's the kind of person who starts screaming when I hug my sister, and tell her I love her. He gets even worse when I hug my best friend who's closer to me than my real sister, even when I explain it's in a platonic way. What should I do?