I have one older brother, and then a plethora of step-siblings. Both of my parents are remarried.
My brother is very much like me, but he's almost nine years older. Our relationship is probably more like uncle-niece, I think. One of the biggest indirect things, though, is that I got to see the differences in how we were treated. My brother is my mom's favorite, you see. He had issues in school (he turned out to be mildly dyslexic, which no one really acknowledged for years), while I breezed through. Thus, he got rewarded when he got good grades, while I was just expected to. In high school, he saved up some money for a car, and my parents paid the rest of the amount, which was more than three quarters of the cost. I was allowed to borrow my mom's car.
Of course, my brother points out that I got a free college education instead, which I admit is a fair trade from my perspective.
My stepdad's kids are all considerably older than me and thus adults when I first met them, so I really don't know any of them well.
My stepmom's kids are all younger - one, three and five years younger than me, I think. I was twelve? Thirteen? when my dad remarried. I find the one that is my age a waste of cellular material. He had to take summer classes to finish high school, knocked up his just-out-of-high-school girl friend at 23, and ten years later is still doing jobs like security guard. Has that had an effect on me? I don't think so, except giving me something to complain about when I get a new status report. The other two I'm mostly meh about.