It may be that they got more vitamins and nutrients from your mother during pregnancy. Traditional cultures didn't allow a woman to give birth to more than one child every 2-3 years and before, during and after pregnancy women were fed the most vitamin rich, nutritious foods available to make sure the child got a sufficient amount of vitamins.
Often genetics are blamed when, in fact, it's a lack of nutrients during the early years that prevent the body to work at maximum efficiency. For example, a deficiency in vitamin A has been shown to cause deformations.