Formal:
* J'ai deux frères et une sœur. (This is the most common and grammatically correct way)
Informal:
* J'ai deux frangins et une sœur. (This is more colloquial and uses a slang term for "brother")
You could also say:
* J'ai une sœur et deux frères. (This changes the order of the sentence and focuses on the sister first)
No matter which option you choose, it is important to pronounce the words correctly. Here are some tips:
* J'ai is pronounced like "jay"
* deux is pronounced like "duh"
* frères is pronounced like "frayr"
* une is pronounced like "oon"
* sœur is pronounced like "sur"
You can use these options to express your family structure in French!