Formal:
* J'ai deux frères et quatre sœurs. (This is the most common and literal translation)
Informal:
* J'ai deux frères et quatre sœurs. (Same as the formal version, but more casual)
* J'ai deux gosses et quatre meufs. (This is a more slangy and informal way to say it, meaning "two guys" and "four girls".)
Important Note: "Gosses" and "meufs" are very informal and may not be appropriate in all situations.