Well I’ve heard both but there is no “cousin” in the link you gave.
And I strongly agree with you that it is a stretch beyond reason.
When I first heard of this belief, here on FR, I realized that the reason they were making such a stretch was because some other belief they had depended on those not being His brothers. I’ve since seen a lot of that sort of thing from others as well as myself: Interpreting a scripture in a goofy way because if it means what the plain language means, some other belief I have has just been skewered or called into question.
Yep! Been there done that but when it kept causing me to run headlong into another passage that didn't fit I finally let go of what I had been taught and used scripture to interpret scripture instead of some guy somewhere who had a "degree".