Edoardo Mapelli Mozzi showed support to Eugenie and Jack Brooksbank by leaving a message on the royal's belated Valentine's post.
The property developer, who married Princess Beatrice in 2020, commented two heart emojis on the intimate family photos.
It comes just weeks after Edoardo and Beatrice welcomed their newborn baby Athena, who was born on January 22.
Edoardo often shares support with friends and family on social media, recently commenting on a photo of four royal children posted by Princess Sofia and Prince Carl Philip.
Edoardo Mapelli Mozzi showed support to Eugenie and Jack Brooksbank by leaving a message on the royal's belated Valentine's post
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The York sisters both have two young children, although Edoardo has a son named Wolfie from his previous relationship with Dara Huang.
The sisters support charities together and have been seen together visiting their parents, Prince Andrew and Sarah Ferguson, at Royal Lodge.
Speaking on the family's close bond, Fergie previously said: "I am very proud of them, and they are very proud of me. We are a tripod of women".
Princess Eugenie and her husband, Jack Brooksbank, split their time between Portugal and Ivy Cottage in the UK, while Beatrice and her family live in the Cotswolds.
Sarah Ferguson and with her daughters, Beatrice and Eugenie, have a close bond
GettyPrincess Eugenie posted the collection of never-before-seen images on Saturday, including new images of her two young children.
Alongside the photos, the princess wrote: "My valentine!! Love you to the moon and back. Happy Valentine's Day (for yesterday...whoops)."
The six-image collection opened with a selfie of the royal and her husband bundled up in cold weather gear, both smiling at the camera.
It included an image of Jack walking hand-in-hand with August along a stony path, the young royal sporting red Wellington boots, a white shirt and denim jeans.
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Another touching snapshot captured a group hug between Eugenie, Jack and their youngest son Ernest.
The princess also showed off a personalised jacket with "Mrs Brooksbank" embroidered on the back.
Several intimate photos featured the couple gazing lovingly into each other's eyes.
The images offer a rare glimpse into the family life of the Princess of York and Jack Brooksbank, who married in 2018 and now share two children, August, four, and Ernest, one.
