Yes and no. It is bit complicated. Some disorders are combinations of variants of different genes, so there it is not the cause. Some are caused by dominant gene variants, so there as well, relatedness does not play a role. But then there are the recessive disorders, where you must inherit similar bad variant from mother and father, so there relatedness does matter.

Close relatedness of parents increases the proportion of genes, where the child has two identical variants. If some proportion of the genes are coding recessive disorders, one is more likely have an disorder. It is the mutation or the genotype that causes the disorder. Relatedness has impact on the genotype. So relatedness can have an indirect impact, but that depends on the existence of those mutations or gene variants.

Most disorders are of the recessive type or something that was beneficial in a different environment.

We are all "somehow related".