Nope, Sam does not become a sprite. And technically, Sam doesn’t ‘turn into’ anything in the story. He is merely cursed to shrink. Any other effects that hit him, such as his knack, are all side-effects of the spell, and if he ever complains that he’s not human, that’s just his own fears coming to light – see here.

There has been talk of involving law enforcement in a future story in the AUs, so it’s perfectly possible you will see this happening down the line! It’s which AU it happens in that’s the hard part to decide, there’s so many different possibilities to bandy about.

Also, Jacob’s stepdad is a cop. So… have fun introducing Dean to the fam. Just sayin’, Jake.

Now all I can see is a wee baby Sammy getting cursed to be small! That poor kiddo, not knowing he has a big brother out there and Dean growing up completely on his own. It’s hard enough on Dean to lose Sam when he’s ten, here he’d spend his entire life without him.

Nope, this won’t be the mystery AU, but it’s certainly a sad idea.

This is the type of thing that all depends on the type of fabric we’re talking about. You can take the kind of shirt I’m wearing and see right through it if you put your eye directly to it. That’s because for the most part girls wear a much thinner fabric than guys do. Now if you look at what the Winchesters wear, you’ll notice that it’s multi-layered shirts that are thick, flannel, dark fabric and the jackets as well will be bulky and stiff comparatively. So Sam might be able to see out of some of Dean’s shirts, but there’s no way he’s going to see out of them constantly.

Unless he feels like cutting himself a hole in the fabric. Then he’ll be seeing regardless.