Re: parents as "cousins" on Ancestry.com #dna
"Endogomy" (Wikipedia): Endogamy is the practice of marrying within a specific social group, caste, or ethnic group, rejecting those from others as unsuitable for marriage or other close personal relationships. Endogamy is common in many cultures and ethnic groups.
This makes the DNA match calculations very complicated.
The well known author Maurice Lamm has an interesting article on Prohibited [Jewish] Marriages at the Chabad.org site that I found very informative:
-Jeffrey Lane, NY