The most famous/infamous, the Duke d'Orléans, Philippe Égalité
Mme Elisabeth, sister of Louis XVI
Maria Teresa of Savoy, Princesse of Lamballe, the great friend of Marie Antoinette
Marie Thérèse de Choiseul, d-i-l of Honoré III of Monaco
And if you step a few centuries back you might have heard of the fate of a couple of Henry VIII's wives and other relatives...
In Portugal even today we speak of the "Távora's bad luck" for a great misfortune, for obvious reasons.
This is not a pleasant subject, but I was just wondering about the notable descendants of Mary, Queen of Scots who met the same fate as hers. The most obvious one would be her own grandson, King Charles I of England and Scotland (executed for treason during the English civil war, pitting the so-called cavaliers against roundheads).
Others coming to mind are the Duke of Monmouth (the eldest illegitimate son of King Charles II), executed at the hands of his uncle. Like his Tudor predecessors on the English throne, King James VII/II engaged in the practice of beheading his rivals -- those relatives who dared question or contest his claim on the said throne. A nephew by blood was not going to be spared, when other monarchs in history were known to kill even their own CHILDREN for treason or rebellion.
Later on, of course, there was the ill-fated Queen Marie Antoinette of France, consort of King Louis XVI -- both being guillotined in the middle of the so-called Reign of Terror, the height of the French revolution.
Are there other cases?