Interesting points on the facial features. Very true and spoken like an artist. That's an interesting angle on the boy possibly being very attached to his sister. If no one is saying "Hey, boys in boys clothes only, little dude.", then a child wouldn't necessarily think anything of it. If he's really attached to his sister, he could easily just be fitting into her world because he loves and respects her.
But there are some parents out there who use things like MA, Boy/Girl Scouts, malls, movie theaters, etc., as cheap daycare so they can have their own lives and time to themselves. I have to wonder if this mom isn't one of those. It'd make sense. She might just want to drop both kids off for the same activity, at the same time, and be the type of person who won't accept "No" for an answer.
Kind of reminds me of the anti-Harry Potter moms brigade mentality.