Lady Sarah Spencer. Their brother Earl Charles Spencer is the youngest sibling. In a recently released Diana biography by Sarah Bradford the Lady Sarah is described as being.
Lady Sarah was the spinster sister of the Red Earl and half sister to Bobby 6th Earl Spencer. The oldest pieces of the tiara are at the ends said to have come from a tiara once belonging to Frances Viscountess Montagu who left the pieces to Lady Sarah in 1875. Sarah Spencer Princess Dianas older sister stars in season 4 of The Crown when she introduces Lady Di to Prince Charles after a brief fling with the royal.
American tabloid claims Sarah is Charles and Dianas IVF daughter.
PRINCE CHARLES met Princess Diana through her sister Lady Sarah Spencer who he was actually dating for a while but the pair had an explosive fall-out that led to their break-up. Says she is the result of a fertilised embryo implanted into doctors wife. In an odd twist McCorquodale is related to Sarahs own stepmother Raine Spencer. Their brother Earl Charles Spencer is the youngest sibling.