Both are correct, but 'a' is not the same as 'one'. 'A' is a grammatical article, 'one' is a number. One means 'not two', 'not three', and so on.
Here's a clearer example. You ask go up to a bar with your friend and say
'Could I have a coffee?'.
The barman hands over two cups of coffee. You point out the mistake and say 'No. Just one coffee, please.'
In fact, "I have a brother and a sister" and "I have one brother and one sister" have the same meaning, but you might use them in slightly different contexts. You might say the first in a neutral context, and the second to make a contrast like this, 'My best friend has seven sisters. That must be strange. I much prefer my family - I have one brother and one sister.'