The Missing Opals

One summer day a couple invited some of their family for tea. Present that afternoon were the couple, their son and daughter, their daughter’s two children, and the son’s wife’s older brother and sister whom the couple had never met before.

Though everyone was trying to make a good impression on everyone else while sipping the tea, later that day a velvet tray of opals was discovered to have disappeared from a cupboard in the family library.

Clues:

During the tea drinking the whole group was together on the verandah, but just prior to this, the following was true:

  1. Three people, all of the same sex and all blood relatives, but each from a different generation of the same family, were in the kitchen making the tea and slicing the lemon bread.
  2. A ten year old boy was training a black dog to jump over a stick on the lawn.
  3. A person and the person’s parent were chatting on the verandah.
  4. Two sisters were looking at the roses in the front garden.

Who stole the opals?