After doing a bit of digging, I discovered that both of Hans Hermann von Katte's younger half brothers, Friederich Albrecht Wilhelm and Friederich Wilhelm Ludwig, died in the same year, 1748. At first I had assumed that they died in battle, but given that the Treaty of Dresden was signed in 1745, I find this unlikely. Does anyone know how they died?
Curiously enough, they both died in a duel with each other. They were both Rittmeister in the Regiment zu Pferde Nr. 9 (9th Cavalry Regiment). Friedrich Wilhelm Ludwig, Hermann von Katte's step-brother died directly, and Friedrich Albrecht Wilhelm died as a result of the wounds suffered in the duel a bit later. There is not much in modern sources about this, and even contemporary sources only seem to mention that they 'died' in 1748. I've only come across a mention of this duel and that it was the reason for the death of both of the brothers of Katte in J. Kloosterhuis, Katte. Ordre und Kriegsartikel: Aktenanalytische und militärhistorische Aspekte einer „facheusen“ Geschichte (Berlin 2011), p. 40, fn. 129. The source for this is given as M. von Katten, Die Katten im Stammbaum : Zum 75. Geburtstag d. Wilhelmsthalers, senioris familiae (Bad Wimpfen 1965), so a genealogy compiled by one of the surviving members of the family, but our Uni doesn't carry it in our library. If you want to know more, that book wold be the place to start.