Everyone makes incest jokes about Adam and Eve and their children but they never mention that there was another woman named Lilith (Adam's first wife) who would have added variance to the gene pool.
If you're grasping at cousins as being the sole separator you know you have a problem. But also there's a lot of conflicting information, such as Cain settling in the city of Nod east of Eden which suggests I guess that there were people outside of Eden already. Don't take anything too literally, and jokes are jokes.
And God got mad at Lilith for not being obedient and banished her so perhaps there were a series of Liliths.
I think Eve was taken from the rib because that was supposed to make her less independently minded than creating her
from the dust like Lilith.