to be honest, I dont have statistics…but common sense would dictate that in communities like these, even if its not a direct 1st cousin marrying a first cousin, there would have been “in family” marriages for generations. If a group of Memons, for example, are all from the same city in India, you’re bound to share the same genes…back in India, they all married within the same town to the same group of Memons, then that same group migrates to Pakistan where they continue to marry JUST Memons from Jetpur, would it even matter after a while if it was just first cousins? I mean you could have a couple that were “distant cousins”, lets say 3rd cousins, whose parents were 1st cousins, and whose parents’ parents were second cousins. There are bound to be some fertility issues there…
But for your particular question, just for the sake of it, lets assume in one family, there have never been cousin marriages and two first cousins marry. I think chance of disease is slim but fertility may prove an issue further down the line if their kids marry their cousins as well.
I think everyone is in agreement at least on one thing…excess of anything is bad, and in this case, its marrying into the family. if this is a one off thing, its not anything to worry about but if its a family trend, you’re better off marrying outside. Heres a good link I found on the topic: