It might not be gluten, but it likely to be environmental, because having kids with a first cousin is not likely to result in any discernable genetic flaw, mental or otherwise. Yes, it increases the probability of such a flaw, but it doesn't increase the chances enough to make it probable. Most of the genetic problems that result from inbreeding happen when there are multiple generations of it, such as when royal families would have closely related couples marry and have kids every generation. Also, brother/sister pairing are way more of a problem than 1st cousins. If it is genetic rather rhan environmental, I think it's likely that the genetic flaw is just due to unfortunate genes, rather than inbreeding. The odds of 1st cousins having 2 full generations in which every sing person has a mental handicap are astronomically low.

Hate to break it to you but first cousin is no big deal. States that allow first-cousin marriages Alabama, California, Colorado, Connecticut, the District of Columbia, Florida, Georgia, Hawaii, Maryland, Massachusetts, New Jersey, New York, New Mexico, Rhode Island, South Carolina, Tennessee, Virginia, and Vermont

There probably is a genetic factor, but the first-cousin marriage bit isn't it. Unless you've had cousins banging for a couple generations, this is just your usual 'abled people pop out disabled lineage' thing.

First cousin does not explain why almost all kids and especially not why all grandkids have mental disabilities. I suggest to really check genetic markers, but also other problems that might be related.

Science says (insert 'Family Feud' wrong answer buzzer sound here). That goes for both of you?