I don't know about other people, but I always gave my best when competing with my younger sibling. I have to imagine the tenacity that develops in such a situation is extremely valuable. They don't look to win, since they know they won't, instead, they look for a series of micro-victories. Which means they learn to constantly analyze their opponents and wait patiently for the opportunity to best them in some small aspect.
I used to teach jujitsu when I was younger. My younger (by 5 years) cousin was a student there and he loved to practice with me outside of class. I remember one time we were doing some informal practice in the afternoon, and during a standing grapple, something distracted me at the door of the gym. In an instant, he grabbed my leg and slammed me hard into the ground. I was totally shocked and he wasted no time jumping up and down, celebrating for a good 5 minutes. He was absolutely as excited as he would have been had he won a championship match.
I'm guessing he had that move planned for weeks or months before ever getting the opportunity to execute it.