While I generally signal when turning, there are times when it is not possible due to traffic conditions. While certainly it would have been more appropriate if the lead rider would have been signaled prior to turning, it seems that there was plenty of time for all 4 bicyclists to turn in front of you and your wife.
Without fully understanding the situation, speed of travel of cars, speed of travel of bicycles, cars moving behind bicyclists in the same lane, etc. it is difficult to understand if this was a dangerous situation for the bicyclists or a situation which you as a more cautious bicyclist are uncomfortable but it was completely safe for the cyclists.