I think she's probably more aware of it than you were at the time, than most people are period. If someone had sat YOU down and explained how rough it would be... would that make it any easier? Or would it make it harder/ put more pressure on the relationship (to break up to avoid hardship, or to stay together/rush things to be 'stubborn')?
I think that commiserating is grand... but there's no need to put pressure on her/them by doing a "sit down" sort of thing... even casually over martinis / etc makes it a "big deal". I wouldn't be able to hold my tongue if it was MY sister... this is just Alice-Advice (I give myself such very good advice, but I very seldom follow it). Reason being, people tried to talk to ME about my LDRs when I was 17-19, and I swore I wouldn't return the favor, and yet... while I know the consequences of it... I'm sure I would open mouth insert foot.
Although... the thing about 18yo long distance is that the VAST majority of the fizzle... absence not making the heart go fonder, but forgetful.
You sound like an amazing sister... I'm sure whatever you say or not, it's going to be the right thing. Not that it won't tick her off, but the right thing in the right way.