Good heavens!

I have no clue where to go with that one!

I know it's not funny, but I have to kind of laugh.
I know in DS's preschool they had a bathroom in the classroom and I saw PLENTY of naked bottoms because they would leave the door open. A former charge was just hopeless at toileting and would walk around with his pants at his ankles at school. But those kids are much younger than Lizzy.
In my almost 15 years of changing diapers I have found that boys' parts tend to splay out and not look like much to anyone passing by, so I am not going withthe diaper story, either.
I don't know if you will gain anything by punishing her for not telling you how she knows, and if she knows what naked boys look like for a bad reason then you don't want to scare her out of telling you. Maybe sit down with her one on one and tell her you are NOT angry, that you were surprised. Then tell her you want to know who has parts like that (that she has seen) and assure her that she won't be in trouble for telling you.
From there you can tackle that it isn't appropriate to draw naked renditions of your younger brother, accurate or not.