Hey I'm a Dane who joined as recently as yesterday.
I noticed this post straight away. I don't know if your questions have already been answered (since the posts are so old) but I will gladly answer them for you anyhow.
Anyway, here's some of the slang expressions you asked for:
big brother is "storebror"
big sister is "storesøster"
(the scandinavian languages have significant differences, so even if the same word is used they spelling will differ. You can't just use the same for swedish or norwegian)
tomboy = drengepige
tough girl = sej tøs
tough woman = sej trunte
powerfully-built male = tyrenakke
powerfully-built female = no idea
cool guy = fed fyr
cool girl = lækker sild
P.S. These are the danish slang expressions. The norwegian or swedish terms are almost certainly completely different.