Verse 33:50,
يَا أَيُّهَا النَّبِيُّ إِنَّا أَحْلَلْنَا لَكَ أَزْوَاجَكَ اللَّاتِي آتَيْتَ أُجُورَهُنَّ وَمَا مَلَكَتْ يَمِينُكَ مِمَّا أَفَاءَ اللَّهُ عَلَيْكَ وَبَنَاتِ عَمِّكَ وَبَنَاتِ عَمَّاتِكَ وَبَنَاتِ خَالِكَ وَبَنَاتِ خَالَاتِكَ اللَّاتِي هَاجَرْنَ مَعَكَ وَامْرَأَةً مُّؤْمِنَةً إِن وَهَبَتْ نَفْسَهَا لِلنَّبِيِّ إِنْ أَرَادَ النَّبِيُّ أَن يَسْتَنكِحَهَا خَالِصَةً لَّكَ مِن دُونِ الْمُؤْمِنِينَ ۗ قَدْ عَلِمْنَا مَا فَرَضْنَا عَلَيْهِمْ فِي أَزْوَاجِهِمْ وَمَا مَلَكَتْ أَيْمَانُهُمْ لِكَيْلَا يَكُونَ عَلَيْكَ حَرَجٌ ۗ وَكَانَ اللَّهُ غَفُورًا رَّحِيمًا
O Prophet! surely We have made lawful to you your wives whom you have given their dowries, and those whom your right hand possesses out of those whom Allah has given to you as prisoners of war, and the daughters of your paternal uncles and the daughters of your paternal aunts, and the daughters of your maternal uncles and the daughters of your maternal aunts who fled with you; and a believing woman if she gave herself to the Prophet, if the Prophet desired to marry her-- specially for you, not for the (rest of) believers; We know what We have ordained for them concerning their wives and those whom their right hands possess in order that no blame may attach to you; and Allah is Forgiving, Merciful.
What I personally understand from the above verse, is that marrying cousins, among others, was lawful for the Prophet, pbuh, only.
Peace,
Khalil