Caught on the baby monitor: Brother, 4, and sister, 2, are melting hearts with bedtime chat

A baby monitor captured a heart-melting moment between two siblings having an impromptu sleepover party.

In the video, Sloan, 4, is curled up next to his sister, Kaia, 2, in her crib.

"Hey Kai, do you want to be my best friend?" Sloan asks.

"Yeah!" she replies.

Sloan then ticks off a list of requirements for being his BFF. Rule No 1: Kai has to behave like a "big girl."

"No crying and no pulling your hair and no screaming and tell mommy the truth," he says. (Note: Kaia twirls her hair at night and it's a habit her parents are trying to break!)

Kaia agrees to the rules, at which point Sloan announces it's time for bed. Kaia closes her eyes.

"Good Kaia," he says, and reaches over to touch her cheek.

After Sloan and Kaia's mom, Liza Lieberman, posted the footage on Instagram, it went viral with more than 3 million views.

"As it was happening, I was like, 'Oh my God, this is all I ever wanted for them,'" Lieberman, a fashion stylist in New York, tells TODAY.com . "My husband and I are both only children and it was very important to us that our kids had a close bond because that is something we always wished we had."

Commenters who viewed the video couldn't get enough of it.

"I cannot get over this!!! This is the cutest thing!!"

"You are an incredible mom and your children are a beautiful reflection of you."

"Most wholesome thing I've ever seen"

"Big brother needs to become a psychologist"

While Lieberman was practically in tears as she watched the scene unfold, she also had a chuckle.

"A lot of what Sloan says in the video are things I tell him, not her," Lieberman shares. For example, when Sloan is having a "big feelings moment," Lieberman will coach him to take a deep breath instead of screaming and crying.

Lieberman also recently had an important conversation with Sloan about lying and why it's important to always tell the truth.

"It's so sweet to watch their bond grow," Lieberman says. "Kaia worships her brother and now that she's talking more he's like, 'Wait a second! We could be best friends!

"I hope that never changes," she adds.