Heidi, 27, is the Duchess's older sister and the daughter of Rainer Luedecke and Jane Ferguson.
On Wednesday, the pleased mother rushed to Instagram to share the wonderful news with her followers.
"My little bunny is engaged to a big bunny called Ben!"
Jane penned a letter. "I'm so happy for them both; welcome to the crazy fun house, Ben."
Sarah Ferguson has expressed her delight at the recent engagement of her niece Heidi Luedecke to art consultant Ben Collinson.
"She is my golden niece, whom I adore," Sarah told Richard Eden of the Daily Mail this week.
"I'm devoted to lovely Ben and Heidi."
Their father sadly passed away in March 2003 at the age of 71, while Sarah and Jane's mother lost her life in a horrific car accident in September 1998. She was 61 at the time.
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