Do the sentences below sound natural?
I have a brother and a sister. He's 16 and she's 14.
Or should you say, 'My brother is 16 and my sister is 14.'?
There are many ways of saying siblings' ages.
All of the above are grammatically correct. The second example is a longer version than the first, but the OP's original sentence sounds more natural because it contains contractions. Just be careful though to never make these typical Italian learners mistakes:
I have a brother and a sister. He has 16 years and she has 14 year * = NO!