'All our most knowledgeable Ulama (Maraaje' of Taqleed) say that marriage between Sayyeda believer and Nin Sayyed believer is allowed as we don't have in the authentic narrations from the Prophet (SAWA) and his Ahlul Bayt (AS) any evidence against it. They have long list if authentic narrations e.g. in Al-Kaafi , an authentic Hadeeths from Al-Hadhrami from imam Ja'far Al-Sadiq (AS) : The Prophet (SAWA) arranged the marriage between his first cousin sister Dhubaa'ah Bint Al-Zubair ibn Abdul Muttalib and Al-Miqdad ibn al-Aswad. This is a clear evidence that Miqdad ibn al-Aswad who was a Mo'min but non Sayyed was been helped by the Prophet himself (SAWA) to marry a Sayyeda Mo'mina who was the first cousin sister of the Prophet (SAWA).

We also have a Hadeeth in Al-Tahtheeb (number 1581) stating the same marriage between the Prophet;s real cousin sister and Miqdad ibn al-Aswad arranged by the Prophetvhimself (SAWA).

Wassalam.