At the service in Hampshire Cressida was supported by her half-brother PR executive Jacobi Anstruther-Gough-Calthorpe.
Cressida Bonas, seen here in December, was very fond of her former stepfather
Christopher was the fourth husband of her mother, society beauty Lady Mary Gaye Curzon. They were married for four years until 2000, during which time they threw lavish parties, including a Venetian-themed bash for her 51st birthday including men on stilts and flame throwers.
Jacobi is one of three children from her mother's second union and he and Cressida are very close.
On arrival at the church on Wednesday, the royal sweetheart was seen embracing the former housekeeper at Hinton Ampner, the stately home near Winchester where the family had lived.
Lady Mary is thought to be on a holiday abroad to mark her 67th birthday next week. Prince Harry, meanwhile, has been supporting a cause close to his father's heart. He joined Prince Charles and Prince William as they attended a conference on how to halt the illegal trade in wildlife.
In general the couple are thought to be supportive of one another, but have so far tried to keep their relationship under the radar.
They were in touch by the phone and email during Harry's stint in Afghanistan at the end of 2012.
At the end of 2013 after the Prince returned from the South Pole with his charity Walking with the Wounded in support of injured servicemen, he immediately met up with Cressie.
The young lovers spent the weekend at the Branson family mansion in Oxfordshire because her half-sister Isabella Anstruther-Gough-Calthorpe is married to Virgin mogul Richard Branson's son Sam.