I've found a picture of the signatures of the royals who attended the burial of HRH Prince Sixte of Bourbon-Parma in the crypt of the Abbey of Saint-Pierre-et-Saint-Paul in Souvigny, on 19 March 1934:
http://i41.tinypic.com/2q9ih76.jpg
From the top, you can see the signatures of:
Princess Hedwige of Bourbon-Parma (née de La Rochefoucauld, widow of Sixte)
Prince Xavier of Bourbon-Parma (brother of Sixte)
Duchess Antonia of Parma (née Infanta of Portugal, the mother of Sixte)
Empress Zita of Austria (sister of Sixte)
............... Archduke Otto of Austria (nephew of Sixte)
.....................................................................Princess Madeleine of Bourbon-Parma (née Bourbon-Busset, wife of Xavier)
Prince Félix of Luxembourg (brother of Sixte)
..........Princess Isabelle of Bourbon-Parma (sister of Sixte)
.....................................................................Prince Louis of Bourbon-Parma (brother of Sixte)
Prince René of Bourbon-Parma (brother of Sixte)
.....Princess Margarethe of Bourbon-Parma (née Princess of Denmark, wife of René)
The Duke of Dodeauville (father-in-law of Sixte)
Grand Duchess Charlotte of Luxembourg (cousin and sister-in-law of Sixte)
Archduchess Adelheid of Austria (niece of Sixte)
Princess Margareta of Bourbon-Parma (née Princesss von Thurn und Taxis, sister-in-law of Sixte)
Prince Gaetan of Bourbon-Parma (brother of Sixte)