Answered by Shaykh Gibril F Haddad
Is one a sayyid even if their family tree stems from one of the other sons of Ali, such as Abbas, rather than Imams Hasan & Hussain?
No, but he may be named a Sharif as the people of Syria and Egypt used to do while the people of Iraq extended the title of Sharif to the descendants of al-`Abbas or to the Hashimis as a whole. However, today, both Sayyid and Sharif are used interchangeably and exclusively for the descendants of Fatima, upon her father and her blessings and peace.
Cf. Muhammad `Abduh Yamani, `Allimu Awladakum Mahabbata Al al-Nabi, Salla Allahu `alayhi wa-Alihi wa-Sallam (p. 26-30).