Baby has heart-melting reaction to brother's kiss

Video of a baby girl's reaction to receiving a kiss from her older brother has gone viral on TikTok, with over 13 million views and counting.

Shelby Wold first shared the video last month, which features her 2-year-old son Theo giving her newborn daughter Lenna, then just 4 days old, a sweet smooch.

At the start of the video clip, Lenna wears a grumpy expression, but after Theo goes in for the kiss, Lenna quickly lights up and then breaks into a grin.

Wold told ABC News that Theo loves to help out with his younger sister, checking in when he hears her crying or going to get a new diaper when she needs one.

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"He was really excited to meet her because we had talked about her for 9 months," Wold told ABC News via email. "I wasn't sure if it would click that she was the one in my belly but he kept saying 'baby sister out now' when we told her she came out of my belly."