I'm not going to invite my extended step family to my wedding
Why don't you want to invite them?
I don't like them very much.
It's your wedding and you should invite people you like and care about. I think it's reasonable to not extend them an invitation.
What do you care what I do with my family?
All family relationships are unique and it's not uncommon for people to not be close to extended step-family members. Ultimately, your wedding is your day and event and you should have a guest list that makes you happy.