It's from "What's Happening To Me?"
My parents gave me this book when I was about 11, and even at that age I knew this was a piece of advice which would get the shit kicked out of you if you followed it. I kept well away from looking at peni until I first saw a porno a couple of years later. Luckily, I had this book so I knew what was happening...
NTA My brother had to be (re)taught how to use deodorant in college. He’d been using it for years but either used it wrong or put it on his clothes and not in his pits? Dunno. I was baffled when he yelled at me for not showing him properly in the first place. I figured my parents had?
Sometimes we all need to just overly show and explain things.
Also the flashing is no worse than him seeing you in a bathing suit.
Your mom either needs to better explain or he needs books on puberty that explain everything. I had this book when I was younger and some of the advice might be outdated but find it or a similar book that’s got a lot of illustrations and it should help your brother understand in an age appropriate way that shouldn’t set your mom off too much. "What's Happening to Me?" The Classic Illustrated Children's Book on Puberty https://www.amazon.com/dp/0818403128/ref=cm_sw_r_awdo_navT_a_JSQSFBFDJRX9QYKN2JJH
When I was 8 my mother made me read this book written and illustrated by 2 men in the 70s. They depicted square boobs and lots of weird stuff. I was so confused. There was even a drawn blonde woman naked with the caption "See, you can be a blonde and brunette at the same time!" Needless to say I was scarred. My mother thought it was funny. I thought the whole thing was cringey and her "sex talks" just became increasingly worse the older I got.
You need a copy of <em>What's Happening to Me?</em>
It's just a really great book for getting comfortable with puberty, and the illustrations are fantastic.
I was thinking more...
> What is sex?
Sex is how a man and woman use their bodies to make a baby grow in the mother.
This is probably all she wants at this time. You should then find a pre-teen friendly book so that she can get the answers she wants.