It is unknown weather Marvin has a brother or sister, because Marcia's parents aren't seen. It is still obvious that Marvin is the youngest of whatever siblings. Her name, after considerable study, appears to be a corruption of the Koshallic name Ma'rwasth'eea, or "Female Spirit of the Double Moonrise". Strangely, she gained her adult name approximately eight Earth years before the general ceremony. However, given the somewhat eccentric nature of her family, it shouldn't be too difficult to allow for extenuating circumstances.