Cousin marriage is still legal in 20 states and in one other one with genetic counseling. Cousin marriage was not uncommon in England in the 19th century including among Jewish families.
I think the key point is, that even though marriage to first cousins is legal in those states does not mean it is institutional and occurs endemically.
However, it is both institutional AND endemic in most muslim communities.
And it has been for a long time. That is a problem.
I’ve read my Bible often and carefully, and I’ve never found a prohibition against cousin marriage in Scripture. If you think about it, EVERYBODY is our cousin in some form, since we all ultimately have the same grandparents in Noah and Adam. So prohibiting cousin marriage, combined with all the actual Scriptural prohibitions, would’ve prohibited marriage completely.
Inbreeding used to be common Sephardic Jews (Jews from Arabic lands). Some communities, like the Syrian Jews, are quite insular. According to an acquaintance, a social worker, the Sephardim have high rates of mental retardation. Ashkenazic Jews (like Yours Truly) are also a genetic time bomb as far as certain diseases like Tay Sachs or cystic fibrosis.
Fortunately the situation is changing, as Jews are losing their provincialism and there is greater "intramarriage" (Ashkenazim marrying Sephardim; absorption of converts.) Also, many religious Jews go for genetic screening before they begin to date (and most of these marriages are arranged).