Hope it does not just confuse but you said there was an older sister "Sissy" (Cecilia? Celia? Elizabeth? The last Empress of Austria, Elizabeth, was always known as "Sissi") who maried a Frenchman. Baptiste is very French, was this a descendent of the French connection and not your person just dropping the name? I think the National Army Museum could help pinpoint if he was ever in the army - or if not at least you would know that for sure. Good luck.