Well,I suppose only June was a relation to the relatives through blood.But you have to remember that there are only four or five different blood groups,so it wouldn't be that hard to find a match for Sheila.It wasn't like DNA testing,mores the pity.
What about Ann? She was a mix of June, through Pam, her mother, June's sister, and RWB. As was her brother, David. As you have pointed out, Sheila's was a common blood group so it was hardly a great surprise to find someone, not biologically linked, with the same blood type.