Our staff did some digging around but did not find anything specific to 3rd generation German-Americans regarding funeral register books. However, we have a few guesses that might explain why only 1 sibling’s name was in the book:

Guess 1: Perhaps some of the relatives didn’t sign because they figured their presence would be remembered or assumed because of their position in the family, so they didn’t think signing would be necessary. Or similarly, maybe the 1 sibling signed as a representative for the others.

Guess 2: Perhaps the book was set in a place that wasn’t easy to see and the relatives missed it.

Guess 3: Perhaps they really weren’t there for one reason or another, maybe an illness kept them from coming.

Guess 4: Perhaps they did sign it, but changed their name afterwards. This guess is based on the fact that many people of German descent did adjust their names to sound more American during and after World War II, although if there was some name changing that happened, you would probably already know that.