Sarah Ferguson says Princesses Beatrice and Eugenie are "phenomenal mothers".
The 62-year-old author - who is the ex-wife of Prince Andrew - has two grandchildren, Beatrice and Edoardo Mapelli Mozzi's daughter Sienna, seven months, and Eugenie and Jack Brooksbank's son August, 15 months, and she's been blown away by how easily her girls transitioned into motherhood.
She gushed: "Most of all, I am marvelling at the brilliance of my girls, who have turned out to be phenomenal mothers.
"It makes me very proud."
The Duchess of York says she doesn't like to "interfere too much" and let her daughters parent how they want to parent.
She continued: "I think as a grandparent, you have to let your children get on with being the parents they want to be, and not interfere too much.
"But I'm always there to pick up the broccoli when it goes flying at mealtimes."
Sarah also revealed that August - who is nicknamed Augie - finds his grandmother "hilarious" when she reads to him.
She told OK! Magazine: "I get quite animated when I read children's stories, and Augie thinks I'm hilarious."
The philanthropist revealed the tots are not yet speaking so she doesn't know what they will call her.
She said: "Sienna is only eight months and August is one, so they're not really speaking yet, and haven't figured out what they want to call me!
Whatever it is will be fine."