If similar traits occur in siblings, it would likely be a coincidence unrelated to Down Syndrome. The genetic basis for DS is well-known: a triple copy of chromosome 21. This is rarely a result of the parents’ own genetic makeup, but results from a defective sperm or egg cell. While there is a rare “partial trisomy” of Chromosome 21 which results in a fewer number of the classic traits, generally you either have full DS or you don’t. And the odds of a family having more than one child with DS are 1 in 100. However, both of my kids, neither of whom has DS, have inward bending pinkies, and one has separated big toes. But theirs are inherited characteristics unrelated to chromosome 21.