Maybe it's some sort of pioneering society on a new continent in the American old west (or a new planet, in the future), and it is only acceptable if Mom is a widow. Then it is the obligation of the eldest son (eldest sounds more exotic than oldest) to continue to bang Mom, as long as she is still in her childbearing years. For the propagation of the society during their pioneer years.
Maybe Dear Old Dad is alive and doing fine, but Number One Son happens to have the major hots for Mommy. He arranges for a series of "accidents" to happen to Dad, killing him off, so that he can spend his nights in Mamma's arms.