I think this is one of the cleverest cases for Miss Marple.
The murderers are so cruel and ruthless. The two murders were committed by Mark, the son-in-law, and Josie. They had married the year before, hoping to inherit the money from Conway Jefferson. Yet, when their prospects were at risk after Conway announced he was going to adopt Ruby and leave his money to her, they decided to act. Josie was the mind-master. In my opinion the most horrible crime was of young Pamela, who had done nothing to this couple. They used her for convenience so that they could have an alibi for Ruby’s murder.
First, they enticed the young girl into the Majestic. Josie made her face up and dyed her hair, and then she drugged her. Mark drove her to St Mary Mead and left her in Basil Blake’s living room. Blake was an occasional visitor of the Majestic and had even danced with Ruby twice. Josie went as far as to mention him as the man Ruby might have fled with, and she even put his photograph in Ruby’s purse. I imagine Mark and Josie chose him randomly just as they chose Pamela. What they didn’t count with was that seeing the body in his place, Blake would carry the young woman to another place. That was why Josie was so surprised to be told that the body had been found in Gossington Hall.
Later when Ruby was still alive, Josie and Mark were seen and stayed in the hotel all the time. Then when Josie went upstairs, it was because Josie had told her to wait for her in her bedroom. Ruby was drugged and fell asleep, so when Josie and Raymond went to check her room,the girl wasn’t there as she was in Josie’s room. Then Josie killed her, and then in the middle of the night she took George Bartlett’s car, drove Ruby to the quarry, and set the car in flames.
Miss Marple discovered the link when she suspected that either Mark Gaskell or Addie Jefferson were married. She checked the records in Somerset House. She also was suspicious about the nails of the woman found in Gossington Hall. It was clear that the woman bit her nails and they were not cut short. So that means that the woman wasn’t Ruby, so if if wasn’t Ruby, how come Josie had identified her as Ruby? That meant that Josie was guilty. The problem was to prove their guilt. So with the help of the police and Sir Henry, they set a trap. Conroy Jefferson told Mark and Addie that he was going to leave his money to a dance school in honour of Ruby. That night Josie was caught on the verge of injecting Conroy with a poisonous drug. The woman was arrested, and Mark, who was a weaker man, confessed to the whole thing.