By Shaykh ’Abdul-’Azeez Ibn ’Abdullaah Ibn Baaz

Compatibilty is only based upon Religion and Piety:

From the evil and reprehensible matters is that some who claim to be from the tribe of Haashim [i.e. claim to be a Sayyid; someone related to the Prophet’s (sallallaahu alayhi wa sallam) family] say that there is no marriage compatibility (kafaa‘ah) between them and someone from outside of their own clan. So they do not get married outside of their clan, nor allow anyone from outside of their clan to marry them. This is a great error, a monstrous ignorance, oppression against the woman, and it is a legislation which neither Allaah nor His Messenger (sallallaahu alayhi wa sallam) legislated or prescribed. Rather, Allaah - the Most High - said:

“O mankind! We have created you from a male and female, and have made you into nations and tribes; that you may know one another, Indeed the most noblest of you with Allaah is the one who has the most taqwaa (piety, fear, and obedience of Allaah).” [Sooratul-Hujuraat (49): verse 13]

“Indeed the Believers are but brothers.” [Sooratul-Hujuraat (49): verse 10]

“The Believers - men and women - are allies and protectors, one to another.”  [Sooratut-Tawbah 9:71]

“So their Lord accepted from them their supplication, and responded: Never will I allow to be lost the actions of any of you, be they male or female. You are one to another.” [Soorah Aali-’Imraan 3:195]