Some states, such as Florida, ban no-contest clauses in wills as against public policy. A greater number of states allow these measures — also known as “in terrorem” clauses — but rarely enforce them. It seems that the law and its practitioners are reluctant to deny people, even disgruntled heirs, their day in court. And there's that maxim: The law abhors a forfeiture.
Unfortunately for a brother and sister, though, other legal norms worked against their challenge to their dad's will.