Ultimately you and your spouse has to present a "united front".
If there is any indication that you are not united your step daughter will play you off of one another. She probably also has her mother's blessing to ignore or disrespect you.
Secondly you want to make sure it's just (you) that she ignores. Oftentimes people take things "personally" but in reality this person is treating everyone with the same disrespect.
I do find it odd though that listed this question under the category of "Gender Relations", "Attracting Women".
Are looking to be involved in her life as a respected adult with authority or do you simply want to become her friend?