Translation:My sister is old, but I am young.
I like that. It seems funny to English ears to call a person "grosso" or "grossa", though, because in English there is the close slang word "gross", meaning "repulsive", or in a lesser but commonly used sense, "extremely large".
But I could do it in Italian since other Italians don't have that preconception.
Very true-- English has the same limitations.
But; in English, there is a difference if you say "This is my big sister" and "My sister is big."
Most people would think size when they hear the phrase "is big", but using big as a modifier before sister instantly makes you think of age.
For latin languages is normal to reduce the compartive adjectives at two simple words "big" and "small" in a lot of situations. But sometimes this practice can blurr the meaning of a sentence. For exemple if you say "the lake is very big" can also mean "the lake is very deep", so it's better to be more specific. This also will show that you really enjoy a language.
Well, I'm a Portuguese speaker. As far as Brazilian Portuguese is concerned, we might say 'ela já é grande' (she is already big). We are likely to use this 'already' to bring the idea of "grown-up". I guess in Italian the adjective Grande also conveys the idea of grown-up, just like Portuguese.