Model, actress and radio personality Pearl Thusi was in a car accident that left her and her daughte.
The South African athlete married Stephanie's brother Albert in a glittering ceremony in the principality earlier this year, and the royal admits she has always been fond of her sibling's now-wife.
Stephanie said: "We've known each other for a long time. We're friends. I like the fact no one knows much about her. They'll discover more bit by bit."
Despite their closeness, Stephanie does not think they are necessarily very similar.
She said in an interview with Britain's HELLO! magazine: "I think it's very important not to draw comparisons, not with me, my sister or my mother.
"Charlene is Charlene. Everyone is different - we've got to stop locking people in predetermined moulds."
Stephanie particularly enjoyed the opportunity to get together with the rest of the royal family of Monaco because so often they meet only in times of grief.
She said: "We've always got together on the big occasions in our lives, whether happy or sad. We can count on each other.
"For us it's harder because when something bad happens to us, it happens in public. There's no mercy." -