Put step dad in his place and grandma's mad now. Parents don't you think I was right to say something to him?

I think you where in the right, no adult no matter how frustrated they are should speak to children that way. The fact that your Grandma thinks it was okay for him to speak that way makes me question how much she loves them. I would as your brother and sister if this was the first time he's acted that way and if they said yes I'd ask your Mom why she thinks it's okay for him to treat her children that way. As a step-dad to young children you should treat them as your own and love them, as a step-dad to older children it's difficult because they're already partly grown but he should still love them unconditionally. You should also ask your Grandma why she thinks it's okay for him to speak to her grandchildren that way, regardless of how annoyed he is with them. It wasn't okay at all.