The 'Roar' hitmaker was by her older sibling Angela Hudson's side last week when she welcomed her little girl into the world, and although she helped deliver her niece 29-year-old pop star still managed to capture the moment on film.
Katy - real name Katherine Hudson - said: "My sister had a baby in her living room - very naturally, not one Advil. I was there kind of being an assistant doula and saw everything.
"We had dinner first, obviously before I saw it all. No, it was just there and it was an incredible experience.
"I was kind of like Martin Scorsese with my iPhone - I have the whole thing. It was like a water birth, the whole thing we're like 'The child is swimming through the water.' It's crazy."
After witnessing a live birth, Katy went on to hail mothers as "real rock stars".
The 'Dark Horse' singer - who recently split from boyfriend John Mayer after 18 months of dating and was previously married to Russell Brand - said: "It's insane, like birth and women giving birth - whenever I see moms walking down the street I'm like, 'Damn, you're a rock star, you're the real rock star - you did something incredible.' "
And Katy admitted her entire family are excited by the arrival of Angela's baby, especially her parents Keith and Mary Hudson.
Speaking on the Fifi & Dave Show on Australian radio station Fox FM, she said: "The baby girl is so alert and aware, and active and incredible. It's the first time in our family to have a little one so everyone's very excited."