Speaking on the red carpet at the British Film Institute Luminous Gala, Sarah added of her daughter: "It's just extraordinary, she's just such a beauty and to see her eyes shining..."
When asked about Edoardo Mapelli Mozzi, who is known as Edo, she added: "I've known him all my life. His brother is my godson.
"I'm so proud, and I've known Edo since he was four, and he's got a lovely mum.
"It's really, really good news."
She said: "I'm really proud of them both."
"And for mums out there, oh my God. I've never understood what it is to have tears of joy because I looked at mothers at weddings, I'm going 'yeah, yeah, yeah, it's good news'.
"No, you really feel it because they're your babies."
When news of Princess Beatrice's engagement first broke, Fergie penned a heartfelt message for social media.
The Duchess shared several engagement photos of the pair to Instagram, with the following caption:
"I know what a mother feels so I have tears of joy. I am so proud of this sensational news. Andrew and I are just the luckiest people ever to have two great sons in law. @princesseugenie took the colour photographs as my girls are so close. Beatrice and Edo wanted her to capture this special moment."