Growing Up Too Fast

I'm the oldest sibling, and I sometimes feel like a parent instead of a sister. My little brother just started middle school, but he's still so young to me. He asked me for help with his homework today, and I realized how much he depends on me. It makes me happy, but also kind of sad because it feels like we're moving away from those carefree days when we'd play outside until dinner time. Do other big siblings feel this way too? Is it normal to want to hold onto childhood while also wanting your little ones to grow up?