"People are free to discriminate based on sex, religion, race, and so on in their wills..."

... but what if, instead of specifying how much specific individuals get, the will says to divide things up according to religious law? Is the court supposed to figure out what the religious law requires? Eugene Volokh has a very interesting post about a case in which the court decided that the sons should get twice as much as the daughters because the will said to follow "Islamic Laws and Sharia." Looking at a Supreme Court case from 1968, Volokh thinks the Establishment Clause requires the court to refuse to make such a religious decision. Volokh also thinks "this rule is right..."

...  even though it does make things difficult for religious people who want the religious terms of their wills and contracts enforced. The alternative, after all, is for courts to take sides in deciding which rival religious view — say, which understanding of Islamic law — is right and which is wrong...

Fortunately, religious observers who want their disputes settled according to religious law generally have a simple solution: They can provide for arbitration by some religious tribunal that they choose, and courts will generally then enforce the result of that arbitration. Civil courts will no longer be called to decide what Islamic/Jewish/etc. law “really” requires, yet religious believers can have their disputes adjudicated under religious principles.

ADDED: Here's a hypothetical with religion taken out of it. A man has 2 children by 2 different women, who are of 2 different races. The will says that his estate shall go to the child who is racially superior.