What I want to say to them is #1 step-children don't like being given orders (the old "you're not my father, you can't tell me what to do!") #2 A much better tactic would be to say "we need to go inside because if we stay out in the rain we'll get a cold, and that will cause [negative effects x, y and z]" ie. rather than give orders, explain why you're asking them to do something