Charlotte, who turned two in May, reportedly burst into tears as she watched George, four, leave to attend his first day at Thomas' Battersea at the beginning of September - perhaps giving an indication of how close the adorable duo are as siblings.
'When George first started school, Charlotte cried when he left,' a source close to the family told Life & Style magazine. 'But she's gradually become used to him not being around in the day.'
The insider added that they'd seen evidence of the sweet relationship between George and Charlotte, explaining: 'There is a lot of banter between the two of them. I've heard George say, "I'm the king of the castle," and Charlotte replies, "So am I!".'
Whilst Prince George looks a little apprehensive on his first day of school, it seems it was his sister Charlotte who had a wobble beforehand
Prince George and Princess Charlotte will soon be joined by a third brother or sister, after parents Prince William and Kate confirmed that they were expecting again back in September.
When the new arrival is born, the pair will remain third and fourth in line to the throne respectively, behind their grandfather Prince Charles, and dad William. Uncle Prince Harry, however, will be bumped down to sixth, one place behind the Cambridge family's latest bundle of joy.
Earlier this month, William said that he thought his second-born Charlotte was set to be 'trouble' when she grows up.
'I think she's going to be trouble when she's older. All fathers say that,' he admitted, although he did continue to explain that: 'George rules the roost and Charlotte isn't far behind.'
During previous appearances, the prince has described George as a 'little monkey' and Charlotte as a 'little joy of heaven'.