For the range, teaching, target shooting, something in a .223 would be nice because of it's low cost compared to a lot of other stuff, plus it doesn't kick bad so your brother and girlfriend won't create a bad habit of flinching. Only problem with it is that it won't be so good for hunting, but from the sounds of it your not too sure about whether you'll be hunting or not. If I was you, the .223 would be my choice over something like a .243, .308 or the 7mm-08 that I mentioned earlier due mostly to price.
"Breathe when you can, shoot when you should."
"Qui desiderat pacem, bellum praeparat; nemo provocare ne offendere audet quem intelliget superiorem esse pugnaturem"