Sarah Ferguson Instagram Sarah Ferguson and Lisa Marie Presley
"I say hello to you everyday and I love you my sissy and I will continue to say hello to you everyday," Fergie, 63, wrote. "You were my sissy, an amazing mother to Ben, Riley, Harper and Finley and a superbly loving daughter to Priscilla. You have been my devoted friend for many years and I am here for your family to support and love them."
She added, "I am deeply saddened my sissy, you are in my heart."
David M. Benett/Dave Benett/Getty Images; Eric Charbonneau/Shutterstock Sarah Ferguson and Lisa Marie Presley
"You stayed with me in my darkest moment and I am here in Yours .. love you my sister Lisa Marie .. wherever, whatever I am with you, in all steadfastness," the Duchess of York shared in the message.
"Dearest Lisa Marie, Finley, Harper and Riley," she continued, naming the singer-songwriter's three daughters. "You have a family here and I love my sissy .. love you to your heart and will never stop loving my sissy."
According to the Daily Mail, Fergie and Presley are believed to have met in 2009, and Presley attended the Duchess of York's 50th birthday party in London the same year.
Todd Williamson/NBC/NBC via Getty Lisa Marie Presley
The daughter of music legend Elvis Presley died after she was rushed to the hospital for a possible cardiac arrest earlier on Thursday.
"It is with a heavy heart that I must share the devastating news that my beautiful daughter Lisa Marie has left us," Priscilla confirmed in a statement to PEOPLE on Thursday evening. "She was the most passionate strong and loving woman I have ever known. We ask for privacy as we try to deal with this profound loss. Thank you for the love and prayers. At this time there will be no further comment."