Just to echo the Mayor's comments, consistency.

Everyone has to be on the same page as to what behavior is allowed and what isn't.

As to interaction with the stepson, just a little story:

My brother had a lab/dane mix. She was a great dog, friendly, playful....except with me when I first came into the house. Not when she first met me, but the first few minutes when I'd come into the house for her entire life, Princess would growl at me. My brother would go to the cookie jar, hand me a biscuit, I'd give it to Princess, and then she'd be fine. Some pups are just odd with certain people.

I'd do classes, with the pup, just to ensure the dog knows who the leader is. It HAS to be you, and with some dogs there can never be a question of who the alpha is (right, Lady?).