Toddler wakes up grumpy -- but everything changes when she sees mum holding baby sister

Babies can melt even the most cynical of hearts, as this heartwarming video shows.

A short clip circulating on Facebook shows a toddler who woke up in a bad mood and in tears -- until she saw her infant sister.

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This definitely surprised their mother, who said she already had a long night and had been expecting another equally long day.

Even sweeter was how the little girl behaved towards her baby sister.

Watch the endearing video below.