And how would you do that? How would you explain the difference when the "difference" is that everything he's told is wrong and bad for him to do because it makes him a sissy are things his sister who's called sissy is allowed to do?

"Boys don't cry that's for sissies. Crying is bad"

"Playing with dolls is for sissys. Playing with dolls is bad stop that"

Meanwhile his sister who is called sissy is comforted when she cries. She's not only allowed to play with dolls she's encouraged to. She's never called Bubba if she plays in the mud. No when she does something that's considered too much like a boy she's "Not ladylike"

It's literally grooming little boys into thinking "Girl's weak, Girls' bad if I do these things girls are allowed to do I'm bad I'm weak"

And the girls see and hear this too. And they're internalizing that.

There isn't a way to articulate "It's different" because effectively it's not. It's literally teaching little boys a negative association with girls. Teaching girls the same thing and teaching girls they're bad if they dare try to act like boys.

It's treating both of them like half their personalities have to be crushed down.

Then as they get older it gets worse. She's bossy, he's a leader. She's a nag, he's commanding.

Trying to articulate how it's "Different" falls apart when it's not. You can either use it as a term of affection or a pejorative. If you use it as both you're making it clear she's bad he's good.