Am I wrong to wonder?

This may sound crazy but all of my life I wondered about my father. My mother got pregnant in her senior year of high school and my parents married shortly before I was born. They had my brother a few years later and divorced when I was 7.
Here is the thing. I look a lot like my mother but stick out like a sore thumb on my father's side. All of the women are under 5feet tall and 90 lbs. I'm 5'9" and 145! I even tower over the men. I'm the tallest female on my mother's side too.
Also, I am the only one with blue eyes, light skin and lighter hair. My brother fits in but I don't. I've never been close to my father and feel no connection to him whatsoever but I don't know how to bring up the subject to my mother.
I don't think that he is my biological father. Is there any way to ever know for sure? I always hear about DNA analysis for genealogy purposes but would that be reliable. Say I take the test and it shows zero Italian but I know my father is 50% Italian, would that be proof?

use your brother's dna and compare it to yours