First, Ancestry DNA's Ethnic predictions are widely held to be inaccurate by the genetic genealogy community. No matter what Ms Cowen says. Ignore them. The distant cousin matches seem to be accurate so use them for confirming your existing family tree and breaking down those brick walls.
Second, we each inherit about 50% of our genes from each of our parents. A sibling can inherit the same 50% or in theory, an completely different set of genes. The overlap is usually around 25% from each parent.