I'm not entirely sure I understand the question. The Fatimids were descendant from Muhammad. They were descendant through Fatima and her Marriage with Ali, and were specifically of the Ismaili line. You can see a partial family tree here: https://en.wikipedia.org/wiki/File:Tree_shia_islam_n3.pdf

Also the premise of your question is incorrect. In Sunni Islam, one need not be descended from the prophet, but merely of Quraysh. It is only the Shia, or Alids more generally, who hold that being part of the House of the Prophet (Ahl al-Bayt) is critical to holding what they more specifically referred to as the Imamate, that is not merely the legitimate political leadership of Islam (whether they actually held political power or not) but more importantly the spiritual leadership.