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.