Most of the time you will say, "I have a brother and a sister." However, you will use "one" when you want to emphasize that you have exactly "one" brother or sister -- not two, three, four, etc. Example: A.: "I have heard that you have two brothers and three sisters." B: "No, that's not correct. I have one brother and one sister."

I think both fine.

Both is ok, the "one brother and one sister" part just puts some extra stress on the number, like if someone ask you what's up with your two sisters? you can answer that "i have one brother and one sister", implying that the person asking the question had incorrect information about your siblings.

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.'

The usage of the word, "one", is just for emphasis.