Here[1]:

> A U.S. citizen can file a petition for the following relatives: • Husband or wife; • Children, married or unmarried A U.S. citizen who is at least 21 years or older may also petition for the following relatives: • Parents; • Brothers or sisters.

From my understanding (and from everything I can find on the USCIS site and all other immigration sites), a U.S. citizen can sponsor immediate family members. They can not sponsor extended family members (cousins, uncles, in-laws, etc.).

[1] https://www.uscis.gov/sites/default/files/USCIS/Resources/A1...