Well,their blood types would be needed to see just who's blood was found in each location.This could be tested in a lab from a sample at the morgue or the pathologist could simply request the victims medical records to see if their blood groups were already recorded.
Either way,I would imagine their blood type would be needed ASAP for the investigation.Then I would imagine the information would be freely available to all investigating officers.
So having ascertained the blood groups of all the victims, and made such available to all investigating officers, how would said officers go about extracting blood samples from relatives who, A) weren't present, and B) weren't suspects? It might make more sense to have taken samples from all police personnel present at the scene. I feel certain that, as Sheila's blood group was so common, at least one of them would have fallen into that group. If nothing else, it may have prevented fingers being pointed at RWB!!!