There's a super cute book called [Morris Mickelwhite and the Tangerine Dress by Christine Baldacchino
](https://www.amazon.com/Morris-Micklewhite-Tangerine-Christine-Baldacchino/dp/1554983479)
My kids (3&5) love it and your story instantly made me think of it.
> I did send each of my siblings a copy of Red: A Crayon's Story, which I'm pretty sure wasn't necessary, but at least the younger kids enjoyed the story.
We have that book too! Another one we have is Morris Micklewhite and the Tangerine Dress
I JUST saw this new book at our library about a boy who wears dresses, and it was so cute. "Morris Mickelwhite and the Tangerine Dress." http://www.amazon.com/Morris-Micklewhite-Tangerine-Christine-Baldacchino/dp/1554983479
It's appropriate (reading level wise) for young kids and normalizes boys wearing "girl" clothes because they like them. It doesn't really get into a trans or gender identity discussion, just more about how dresses are ok for everyone, but I was super stoked to even see something like that for young kids.
Yikes, that's shit, I'm sorry that you're both dealing with this. (By you both, I mean you and your baby cousin.) How close are you to your baby cousin? Because that's probably not a great thing for a child to learn in general, so depending on how you want to play this (or if you do), there are children's books that can either shake the "girls are princesses" and "boys can't be pretty" stereotypes, as well as books that introduce the idea of being trans, all in easy to digest picture book formats.
There are a number of them so this is definitely not an exhaustive list, but for a few examples in the "boys can be pretty" category: Sparkle Boy, Julián Is a Mermaid, and Morris Micklewhite and the Tangerine Dress.
And in the "girls can be <insert masculine stereotype here>" family. Rosie Revere, Engineer, The Paper Bag Princess, and Pearl Fairweather Pirate Captain
And to introduce trans related concepts in a simple child like fashion: Red: A Crayon's Story, and Introducing Teddy.
So if you think it might be helpful, you may be able to find books to use. Other people have suggested saying something to your cousin instead, which might also be a good idea. But it might be a good idea to say something (child appropriate) to her.