"My sister is old, but I am young."
Translation:Mia sorella è vecchia, ma io sono giovane.
Now this sentence popped up at a much earlier level compared to where I am now, and the answer was as given here - using grande and piccolo. When one returns to it in practice, however, things have moved on and we've learnt giovane and vecchia... but these are marked as incorrect. There are some things that just don't work.
Since when "grande" means "old"? And piccolo means young..? Does'nt that mean "big brother" and "little sister" ? Correct solutions: Mia sorella è grande, ma io sono piccolo., Mia sorella è vecchia, ma io sono giovane. I did'nt pick the first one and failed the exersize.. Not fair..
Here is a helpful site for possessive adjectives. http://www.arnix.it/free-italian/italian-grammar/possessive-adjectives-in-italian.php