I was 9 when my step dad became a part of my life. Nearly 10.
Initially I accepted it and was fine. Then my dad started making subtle comments that made me feel I couldn't like my mums new boyfriend and not be disloyal. I start acting out. My Mum and Step Dad persevered through this and got married when I was 12.
To my mind we became a proper family when I was 14.
Today at 24 I refer to my "three parents" and "all my parents". I would not be who I am today without my step dad. I love him dearly.
Step parenting is hard, I've been the difficult step child, I know.
The children of your partner will come around. I never though I would and yet here I am.