i have been on both sides of this as a step kid (he was my moms boyfriend not step dad or me step kid) and a step father.
my mom got with him when i was 10, long story short i was always in "trouble" and got permanently grounded (only allowed out of my bedroom for school, bathroom and washing up) for 18 months till i went to live with my dad. it left me withdrawn and agitated being around people, i prefer to be alone.
as a step dad i swore id never be a cu*t and treat my step kids right, i never acted as a father to them but more as a friend, always there when they needed something.
i got luck with there real dad as he didnt try to cause trouble, after a few years he stopped trying hard with them as he got a new step family.
9 years after i split with there mom they still see me as a step dad and we see each other regularly.
biggest advice i can suggest is dt be a step mum but more a friend, try and find something for just u and the kids, like with my youngest step daughter it was going to a cafe on a sat morning for a fry up,