Funny story. What should I do about little brother

So, I'm Stephanie and I'm 17. Last weekend we went to this lake near my aunt's place, and there were all my cousins (all girls), my sisters (one is nine and the other fourteen), and my brother who's twelve. The afternoon was actually hilarious. I mostly just swam and hung out talking with the cousins that are near my age, since the lake was empty we could swim a bunch. But before swimming, my younger cousins said to my brother he couldn't swim with us, so they grabbed some hair ties and actually tied his hands really high behind his head to a tree. He just went along with it, playing along while smiling like shyly. Then they stuck some string in his mouth and tied it around his head as a gag. He was just wearing his shorts at the time, but then the girls yanked those off him—and he actually tried to fight it off, but they got them off, leaving him totally naked. Then they tied extra strings around his ankles to the tree. Seriously, we all the older girls and me couldn't stop laughing. My brother was writhing and making these muffled complaint noises, but they had him tied up good, so he was super embarrassed. We ended up swimming with him there. Later, we were just lounging on the grass soaking up the sun, my little cousins and my sisters kept teasing him, they were tickling his ribs and flicking his pee-pee. We all couldn't stop giggling at how funny it was. He was just standing there in all his skinny glory, trying to move his hips around so he wouldn't get hit, and his little pecker is so tiny that we couldn't stop laughing, and the girls couldn't stop making fun of it. Anyway I’m still laughing about the whole scene, but I also kind of feel worried about him. He’s twelve already and he has zero body hair, like not even under his arms or anywhere down there, he’s super smooth and hairless. Plus, his bits seem way smaller than what I expected for a 12-year-old. It was a fun time, but also he's my little brother and I love him, I'm wondering if I should just ask mom about whether or not this is normal development stuff, or if there's to worry about?