Rumer Willis is the best big sister ever!

The 28-year-old took some time out of her busy schedule to hang out with her much younger sibling Mabel — and the pair got together to sing an original song for her Instagram followers.

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"I was never this cool when I was 4," she captioned the cute video. "My little sis is my ❤. Mabel I love you so."

Rumer was born almost 14 years before her half-sister, who dad Bruce Willis welcomed in 2012 with wife Emma Heming. Bruce also has daughters Scout, 25 and Talulah, 22, with ex wife Demi Moore, and Evelyn, 2, with Emma.

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Too cute! For more celebs with much younger siblings, check out the gallery below!