None of the tiaras Princess Margaret wore, except the Poltimore, were sold at auction. I think it is likely that the lotus tiara never belonged to Princess Margaret, but was always the property of the Queen Mother, until the Queen inherited it. There is the odd picture of one of the Queen Mother's employees with the tiara askew on his head. I think we have surmised that it had just been retrieved from the vaults, very likely at the last minute if the Poltimore tiara broke, for Serena Stanhope's wedding. The family tiaras Margaret wore may all have been loans from her mother. The lotus tiara was originally the Queen Mother's private property (starting when she was Duchess of York), and therefore it is not part of the jewels reserved for the monarch. It stands to reason that, like the Duchess of Cambridge's wedding tiara, these tiaras are available for her to wear.