Estate tax

People who support the estate tax (I don't, but that's beside the point) tend to say that the estate tax is good because it prevents a lot of wealth from concentrating within the same family.

So why is it that both the estate tax law and the inheritance law try to prevent people from leaving their money to heirs outside their family? If this society wants money to circulate, rather than to stay in a family, what is the point of charging double or triple estate tax when people inherit from distant relatives and friends? And what exactly is the point of the law that says that half of one's estate goes to one's children?