Marrying your cousin is usually seen as a sign of not being able to find a spouse independently. In the more populated and diverse southern regions of Somalia, cousin marriage is less common. Plus, these regions are far from the Gulf, where cousin marriages are more prevalent. So, the previous comment may not apply here.
Cousin marriages are not really common in all of Somalia and are more individual cases not in high enough numbers to make sweeping statements where and where not its more common.