If I remember right the chance of birth defects with cousins is around the same as women in their late thirties. Probably isn't that much worse for siblings, the risk comes with generational incest.
So yeah, go fuck yer sister man.
I care less about the Ein thing and more about Alice wanting to be Luna's knight so badly.
Also, Alice is "top of the class" yet Yuno got the first ever perfect on their written exams.