I agree with both replies. It can even become further complicated with blended families and two "only children" you cannot imagine or maybe you can. It is true the Step siblings will do many unusual behaviors even after they "grow up" and move out. At least this has been my experience. My pipe dream and reality have collided into the reality of what is, and how very complex life and living are these days.
Some days are better than others. With my H son, I have known him since he was 10 and now that he is 24 we give each other space, and respect, also I know his habits background and "tricks" when he "acts" I call him on it and remind him where and who we are today. I am like a mentor to him, but we are not warm and fuzzy close.
My H has a "respect" first for my son and vice versa, my son is 36, so they relate more on equal respectful footing, but not buddies or pals. This works well so far. It is ongoing, building and learning with and through for us.

I hope this is somewhat helpful. I know there are lots of families who struggle, today is a challenging time to live in.

I wish you well, and good luck with your
"step children" love them as best as you can it is a tough spot to be in.

Jade

I