I am the youngest of 3, and my two siblings were 5 and 6 years older than me. While they did not beat me up, I was belittled a lot, corrected a lot, and treated sort of like a cute but dumb kid a lot. As an adult I have always gravitated to other youngest children (I also married one!) and very much dislike being "bossed" or controlled or told what to do. If I sense a superior-inferior dynamic in a relationship, I am outta there.
There were so many other issues in my family that this sib stuff has always seemed minor. Still, it is obvious to me that it's affecting my parenting negatively. I don't know how to process this and it's also hard for me to discern what is "okay" sib fighting and what is bullying. DH has similar issues, so he is no help.