You can go to Chinese embassy in Tehran to apply for a Tourist Visa (L visa), if you can't come to the embassy, you can ask someone else or travel agency to apply for visa on your behalf,
as for the requirements:
1. invitation letters from his family
2. A passport valid for more than six months before expiration and at least one blank page in it.
3. One complete application form (The People's Republic of China Visa Application Form) with one recent passport-size photograph (black and white or color) glued thereto.
4. round-trip flight tickets
5. copy of their marriage license, the relationship between you and your sister.