they are 100% cotton, yeah. they stretch about a size, sometimes more if they're really tight.
it depends on how much you plan to grow. I was a true 29 when I first got my diors and I ordered a 27. wish I ordered a 28, because they were really snug when I first got them, and I have since gained weight and had to sell them. supposedly the nostalgy (mii) stretches more than the brut (mij), but that shouldn't really affect your sizing choice. honestly, if I was you, I would order the 27. that way they'll be comfortable right when you get them, they'll stretch a little but not too much, and you'll have room if you end up gaining weight, etc. you could always exchange if you really needed to, but in my experience I found that sizing down 2 (a 26 for you) ends up being too tight, and you don't have much room for growth, if any.
hope this helped.