Zac Efron melted the hearts of his 56.7 million Instagram followers on Monday when he posted a birthday tribute to his little sister Olivia.The 33-year-old heartthrob – who sent pulses racing earlier this month when he worked out shirtless – posted a pair of sweet pictures that showed him holding his adorable 2-year-old sibling.Olivia appears to be the daughter of Zac's father, David Efron, and his second wife, making little Olivia Zac's half-sister. Melting hearts: Zac Efron, 33, melted the hearts of his 56.7 million Instagram followers on Monday when he posted a birthday tribute to his adorable 2-year-old sister Olivia'Happy bday lil sis,' wrote the High School Musical star, captioning an adorable pair of images. The first picture shows Zac holding Olivia as she gazes up lovingly at his mustached face of chiseled features, which have caused widespread speculation that the star has received plastic surgery.The little girl's flower headband kept her golden curls in place as she marveled at her big brother, who was wearing a festive green button-up.Zac's clean-cut hair was lightly styled with gel, giving him an effortless look that paired well with his rugged appearance. Always tanned to perfection, the Baywatch star sported a sexy holiday glow. Cuteness overload: 'Happy bday lil sis,' captioned the High School Musical star. The sweet pair of pictures showed Zac holding Olivia as she gazed up lovingly at his mustached face of chiseled features before snuggling into his chest as he kissed her on the headIf Zac's first picture didn't make his fans go weak-kneed, then his next picture certainly went in for the kill.Olivia snuggled into her big brother's chest as Zac clutched her close and kissed her head. 'How is this man