The author, 63, posted three images – one showing her late mother Susan Barrantes with Her Late Majesty, a second of a young Sarah with her future mother-in-law, and a third of a close-up of Susan beaming and holding a telephone.
"My beloved mother Susie died 25 years ago today, taken far too young. There isn't a day that goes by when I don't think about her, her indomitable spirit and her joy for life," Sarah wrote in the caption.
"Today is also the first anniversary of Her Late Majesty the Queen's funeral. As I have often said before, the Queen was like a second mother to me and I will always be grateful for the love and understanding she showed me throughout her life.
"Today I celebrate the legacy of the two women who gave me so much strength and courage. I am so thankful for their guidance and the examples they set, which I try always to live up to."
Sarah's mother Susan died in a car crash at the age of 61 in Tres Lomas, Argentina on 19 September 1998.
The documentary filmmaker shared two daughters – Sarah and Jane – with her first husband, Major Ronald Ferguson.
Susan later married Argentinian polo player Hector Barrantes in 1975, following her divorce from Ronald the previous year.
Meanwhile, Sarah's former mother-in-law, the late Queen Elizabeth II, died at the age of 96 at Balmoral Castle on 8 September 2022.
Her Late Majesty's funeral took place at Westminster Abbey on 19 September, followed by a committal service at St George's Chapel at Windsor Castle.
Sarah, who still lives with her former husband, the Duke of York, remained close to the late Queen after her divorce from Prince Andrew in 1996.
The anniversaries come just one day after Sarah's granddaughter Sienna's second birthday.
Princess Beatrice's husband, Edoardo Mapelli Mozzi, shared a rare photograph of their daughter to mark the occasion on Instagram.
Meanwhile, Sarah attended The Perfect World Foundation Awards in Gothenburg, Sweden on Monday night.
She appeared on stage with Sex and The City star Kristin Davis as the pair presented an award.
Listen to HELLO!'s A Right Royal Podcast and discover the surprising shoes the late Queen wore under her robes...