Yesterday, Prince Guillaume and Princess Sibilla attended a memorial mass for Prince and Duke Ernest-Charles d'Arenberg who was killed in battle on March 21, 1915, fighting for France during the First World War. The memorial mass took place at the Cathédrale Saint-Louis des Invalides in Paris. The scion of the French branch of the family was married to Thérèse de La Rochefoucauld d'Estissac, who died in 1958. According to this 1915 New York Times article he was the 10th member of the French aristocracy to die in World War I.
Getty Images has a gallery of visuals of the mass also attended by a number of other royals and nobles.