yes
Romeo, his parents, and his cousin Benvolio are the only members of the family we meet. There are also some servants like Balthazar and Abram.
No they live in a storybook
In Romeo and Juliet, Romeo is the sole heir to the Montague fortune. This means that he must be the only surviving son (the play suggests that he is the Montagues' only living child). We do not know if Romeo was the eldest son, but we do know that he is the only one who is still alive.
David Beckham has called his third son Romeo.
He doesn't have a sister. He is an only child.
boy
No they are not.
David Beckham
Capulet
The Prince calls Romeo Montague's heir suggesting that he is his eldest (if not only) child. See V iii 209
He learns that Juliet is the only child of the Capulets and rich.