This is extremely suggestive

Dating your brother in law sister

As you can see above iterated generations of inbreeding tend to result in some serious problems. The chart to the left plots out the process of repeated brother-sister matings within an isolated population over time. The Spanish Hapsburgs had a higher infant mortality than Spanish commoners.

Consider if you have a dozen

Though in many cultures cousin-marriage is preferred for various reasons, until recently it was extant at lower frequencies than is the case today. Not only did the Spanish Hapsburgs have a high infant mortality rate, but it got higher as their inbreeding become more advanced. His brief life consisted chiefly of a passage from prolonged infancy to premature senility. But this a story of genetics as well as history, because historians have long assumed impressionistically that there was something rotten in the gene pool of the Spanish Hapsburgs.

Consider that lower infant

Consider that lower infant morality in modern societies, and the large families the norm in many developed societies. Consider if you have a dozen first cousins to choose from vs. Over time this sort of process would result in radically increased inbreeding coefficients. But it is nice to see it laid out in a precise manner. In the case of the Hapsburgs there is no need for de novo mutations, all humans carry within them numerous deleterious alleles which are masked because they only express recessively.

Assuming that the grandparents, of whom there are only two, are unrelated. If drift becomes powerful enough it can lead the population toward extinction through mutational meltdown. In plain English, the higher the inbreeding coefficient the higher mortality.