Lady Sarah is actually close family to the Queen: she's the daughter of Queen Elizabeth's late sister Princess Margaret. After losing Margaret in 2002, the Queen became especially close to Lady Sarah. She keeps an eye on the Queen, who still carries out plenty of duties despite already celebrating her 92nd birthday.
Like her father, the photographer Antony Armstrong-Jones, Lady Sarah is an artist; she's a painter using the nonroyal name "Sarah Armstrong-Jones." Another fun fact: she was briefly referenced in the second season finale of The Crown, when her mother Princess Margaret is pregnant with her second child; that child would be Sarah.