Rumer Willis's little half-sister Mabel is full of sass! The adorable 3-year-old, who is the daughter of Bruce Willis and his second wife, Emma Heming, wished Rumer a happy 27th birthday [2] earlier this week with an adorable Instagram video. In the short clip, Mabel wears a large hat and a purple Hawaiian lei while carrying an orange bucket. "Happy birthday, Rumer!" she yells into the camera. "I love you!" Rumer, whose big day was on Aug. 16, was obviously thrilled with the message, writing, "Best Birthday Message Ever courtesy of my beautiful sister Mabel." Watch Mabel in all of her glory above, and then watch Rumer's parents, Bruce Willis and Demi Moore, get emotional about their daughter in this sweet video [3].