My parents are having a baby!

I'm 21 years old and I'm going to be a big sister for the first time. (My dad and stepmom are having a baby) I am still so in shock! (But a happy shock!) I have a 5 year old step sister but this is different! They just found out last night. I'm so so happy! 
But anyways, just wondering if anybody else has a huge age gap between you and your siblings?? In highschool, I had two friends who's parents got pregnant our senior year so they're 18 years apart. It's just so weird to think about. My husband and I are going to start trying for a baby next year. My kids are going to be around the same age as their uncle or aunt. Lol. But my dad is only 45, and I think my stepmom is somewhere between 36-39ish. I'm happy for them, just so shocked! I want my brother or sister to view me as a big sister and not an aunt or something since I'm so much older. Any advice on how to be the best big sister ever? 😁