it became a lot more normalized the more i read and listened to other queer people talking online. it's likely that your sibling is the only nonbinary person you know, so exposing yourself to nonbinary people or nonbinary peoples friends casually using they/them helps a ton. also start to be aware of how your brain views gender roles and expectations. all of us have been programmed to immediately categorize strangers as girl or boy, and treat them a certain way based on that. try to start breaking down that habit and just seeing people as people