Dating advice please help:)?

I really like a boy that i went to preschool and kindergarden with. We are now Freshmen. Me and his sister (she's 11) are pretty good friends. We all go to church together, but all different schools. I dont talk to him all that much, but i really like him. Would it be okay for me and him to date even if me and his sister are friends? And if so, How should i ask him out?