Diwana is right, it’s allowed but NOT encouraged. ‘Keep in mind’ does not mean it’s preferable or encouraged to do so. This is culture. In fact the opposite is true, it’s preferable to marry outside.
'The Islamic view is that while marriage between cousins is permissible**, it is preferable to choose a marriage partner from outside one’s family. **We have to distinguish between what is permitted and what is advocated. Some clans restrict marriages to amongst their kin only – a practice far from what is advocated. It is worth stressing here that when marriage of cousins is repeated over several generations, they are bound to have more effects on children.
By permitting such marriages Islam does not encourage them. It advocates the cementing of social relations through marriages between totally unrelated families.
The Prophet (peace and blessings be upon him) once told one of his Companions to choose a wife from a tribe different to his, and then to choose for his son a wife from a third tribe, and to seek for his second son a girl from yet another tribe.
Preferring this course of action, Islam nevertheless permits marriage between cousins because it meets a social need.”’
Source: Islamic View on Marrying Cousins - IslamonLine.net - Ask The Scholar
If everyone married within their family how would Islam ever spread..