No need for family separation. You simply compare the correlation between monozygotic ("identical") twins vs dizygotic ("non-identical") twins.
For example, monozygotic twins will always have the same eye color (99+% correlation), while dizygotic twins do not. Thus we can conclude eye color is genetic. Both twins are raised by their respective parents, so it's unlikely parenting is causing this difference in eye-color-correlation.
For example, monozygotic twins will always have the same eye color (99+% correlation), while dizygotic twins do not. Thus we can conclude eye color is genetic. Both twins are raised by their respective parents, so it's unlikely parenting is causing this difference in eye-color-correlation.