Uncle
My brother is eight years younger than me, and when he was little (and much shorter), people often assumed he was my kid. I was often in charge of him, maybe a benefit of the age gap or just because of my personality. 😉 It's weird to think that he's now 24, the same age I was when I moved to Seattle for a job at the P-I.
My brother has been the world's best house guest: doing the dishes, cooking, laundry... It's a huge treat to have someone else taking care of me! Babies aren't really his thing, but Joseph is 3 now, and he talks and plays and does stuff, and my brother has been the nicest, most patient uncle to my little boy.