I am a step-kid. My mom remarried the first time when I was 7 then my stepdad passed away after a number of years. She is currently married once again to a really great guy. One of my first negative memories of my first step-dad (the one that still sticks with me today) is him trying to discipline me right off the bat. I was very resistant as he was not my biological father though I'm sure it varies from kid to kid and depends on age too. His kids were grown and out on their own and we never formed a "sibling" bond.
Currently, my mom is a step-parent to my step-dad's 3 children (they live out of state). They are ages 11, 16, and 19. For the holidays, my mom welcomes them with open arms and with a big smile on her face. The kids are mean to my mom after she bends over backwards to provide whatever they needed to be comfortable for the holidays. My mom has never tried to fill in as their mother because they obviously already have one. I have zero tolerance for anyone being absolutely disgusting to my mother so there have been times where I told them their behavior and language is unacceptable. I love my step-siblings, but being a blended family does not give anyone the right to be full out disrespectful especially if they are old enough to know better.
As for my relationship with my current step-dad..it's great! We get along really well and whenever my mom and I end up in an argument, he never swoops in and takes her side even if she wants him to. He will llisten and give his opinion if we ask, but will remain neutral, which I love. I recommend this for other step-parents.