An interesting legal question (and, as usual in such cases, hints of a deeply unpleasant factual back story), from In re Estate of Feinberg: "Can an Illinois court enforce a testamentary provision that any of the testator’s grandchildren who marry outside the Jewish faith, unless the spouse has converted or converts within one year of the marriage to the Jewish faith, will, for purposes of the testamentary instrument, be deemed to be deceased, along with all of his or her descendants?"
The court says no, by a 2-1 vote. Two judges take the view that such a testamentary provision is against Illinois public policy, and courts should not enforce it. One of the two judges in the majority also argues that it's unconstitutional for the courts to enforce it, citing Shelley v. Kraemer, the 1948 case that held that courts may not enforce racially restrictive covenants. The dissenter disagrees on both counts, and suggests that the provision is a legally acceptable way for the decedents to "seek to preserve their 4,000-year-old heritage." The judges acknowledge that the precedents from other states are mixed, though those precedents all tend to be quite old.
The judges do not expressly discuss a different possible constitutional objection: That the Establishment Clause bars civil courts from deciding who is within "the Jewish faith," just as civil courts may not decide which church is truly Presbyterian and which has "depart[ed] from [the orthodox] doctrine." In at least some situations, there would be serious disputes over who belongs to "the Jewish faith," especially since that term usually means "is Jewish under Jewish law," a matter that can be famously contested among the different streams of Judaism; I doubt that civil courts are allowed, under current First Amendment law, to resolve such disputes. Should that preclude any judicial enforcement of such conditions in wills, or only when there is in fact a serious dispute about whether the spouse is Jewish?
(In case you don't get it, an explanation of the post title can be found here and here; it's a pretty famous joke among Jews, I think, though the attitude it refers to is more sad than funny. Setting aside the propriety of the grandparents' underlying plan, I would have hoped the trust could have been drafted in a way that's less symbolically unloving than labeling some grandchildren as dead, even if such labeling is convenient as a matter of pure drafting.)
Thanks to Michael Pitkowsky for the pointer.