Answering the topic question: How much of an effect does parenting style have on a child? My answer: 100%. That is to say that 100% of parenting style has an effect, not that parenting style determines 100% of personality.
Answering the question at the end of the text: Is there any evidence that the parenting makes a big difference on who the child becomes? My answer: Absolutely.
For proof of these answers, I can only submit my own experience.
I was telling people the last month or so that the best thing my parents did was move from our home town. Most of my family that lives in that city turned out to be shiftless, drug-addled, ambition-free nobodies. I am convinced that growing up nowhere near my dropout older cousins saved my life.
These are people that had no strong male figures in their lives providing either direction or example. Mothers were more of a very sweet housekeeper than mother and fathers were harsh and simply not interactive. This lack of parental direction led them to shortened high school careers and a lifetime of squalor. As directionless as my own upbringing was, had I been local to these people during my hs years (when I was bored with the books and growing responsibility) the temptation to go hang with my cuz and smoke & drink may have been too tempting.
Conversely, as I've said a number of times around here, I can see a measure of success of my method in my reasonably successful 20 yo and it gives me confidence as I deal with my 18, 10, 8, 6 yo's. At 20, he's got more ambition and direction than I had till I was probably 30. They're all different personalities and require different approaches, but that simply comes with the territory.
I don't just give lip service when I say I love my kids. I interact, play, teach, chill, etc etc. I give them focus, family and friendship. I give them roots. And as they get older, I give them wings. I got none of this coming up, and I believe that because I was just generally in a better environment than my cousins (suburbia of a midwest city as opposed to the urban center of a large southern city), once I had the opportunity to make my own choices (mid teens) I didn't make the wrong decisions. I had time to figure things out on my own without anybody pressuring me.
Your experience with the difference between siblings can be explained, in my opinion, by simple parental favoritism. My 2 oldest, when they were kids, felt the other was the favorite. They're both step sons, and i get that it's absolutely normal. The younger set does not have that anxiety as avoiding outward favoritism is a huge part of my method... why? Because I was/am not the favorite and I know how much is sucks. And when a person discovers that fact, it's destructive... whether it's the parental relationship, the persons outlook on life, or whatever.
Preemptive Disclaimer: To anyone that wishes to correct or redirect my statements using actual citations, I'll just say that I'm an adult sharing his experiences. I'm not educated in the finer and more abstract aspects of the nuclear family dynamic. Say what you will, I'm not easily offended.