Do you have a sister?

Yes, I do. I have a brother and a sister.

Do you have any brothers and sisters?
Yes, I do. I have a brother and a sister.

Which answer is correct? Are both answers correct? Which one is natureal?

They are both correct. The second one is better, because the first question only asks about a sister.

Both answers are correct. The first answer implies that I have one sister, while the second answer indicates that I have both a brother and a sister.

The second answer is more comprehensive because it includes information about both siblings. However, whether one answer is more natural than the other depends on the specific context and the information being conveyed.