Nicole Kidman shares a sweet childhood photo alongside her lookalike sister Antonia as she pays tribute on her 51st birthday

Hollywood megastar Nicole Kidman is known to be close to her mother Janelle and her lookalike sister Antonia. 

And on Wednesday, the Academy Award winner shared a sweet tribute to her younger sibling as she celebrated her 51st birthday.   

Nicole, 54, posted a flashback photo of the pair from their childhood and wrote: 'Love sharing this life with you.'

'To my darling sister - happy birthday,' she added, alongside a cake, love heart and star emoji. 

In the image, both Nicole and Antonia pose by the water and wear matching shorts, T-shirts and hats. 

In October last year, Nicole revealed just how close she is to her sister. 

During a chat with WSJ Magazine Editor in Chief Kristina O'Neill to promote her show The Undoing, Nicole admitted that she and Antonia have their own 'secret language.'

The topic came up after Kidman was asked about her connection to co-star Hugh Grant.   

Recalling how they first met over two decades ago, the Big Little Lies talent said: 'I've known him since we were in our early 20s. 

'We went to dinner at a place in London called The Ivy with his then girlfriend Liz Hurley and my sister and just a huge bunch of people and we just kind of clicked.'

That night Hugh became fascinated with Nicole and her sister's special style of communicating. 

Nicole explained: 'My sister and I were speaking our secret language and Hugh was fascinated, because my sister and I have this language that we can speak so we can be understood in a crowd and nobody can understand us.'

O'Neill prodded for a bit more information but the Moulin Rouge star only laughed saying: 'If I end up telling you then you'll be able to decode it.' 

Nicole called the secret language 'silly' but said it's always come in handy.

'It is helpful, let me tell you. It was really helpful when we were like at parties and they were and we had our boyfriends and stuff we would be able to use our language and they'd all be like "What?"'

'It's a really good tip all the sisters out there,' she went on.