I was just trying to pm you, but you don't have enough posts..
Have a conversation with your wife first, let her know your plans, then have the conversation with your step daughter.
Please don't let her fall in to a trap of measuring her self worth against how badly boys want to have s3x with her. Please, just (respectfully) point out her value and self worth. Put a stop to the texts / photos / whatever now.
As for the discipline, I think that's difficult if you've had 3 years of leaving it up to your wife. But do not allow anyone in your house to disrespect you, you should still be the head of your household.