Speaking to Andrew Bates and his wife Janet, guests at a recent Buckingham Palace garden party, Kate reportedly said that she was 'really looking forward to her sister's wedding', but added that she was a bit worried about how her children might behave.
'She said she was hopeful that they would be good but you never know at that age,' Andrew explained.
Prince George and Princess Charlotte were both be confirmed to be part of aunt Pippa Middleton's wedding party back in April, with Kensington Palace revealing that the young royals will act as page boy and bridesmaid respectively when Pippa ties the knot with financier James Matthews on Saturday 20th May.
It's also been stated that there will be no official role for Kate in the ceremony, although Pippa was maid of honour for her older sister when she wed Prince William at Westminster Abbey in 2011.
Tradition would dictate that Kate would return the favour, but earlier reports have suggested that the mum-of-two is anxious not to 'upstage' Pippa at the wedding, and therefore has been keen to take a back seat.
'It's a tricky situation for Pippa. She would want her sister by her side, as who else would she trust to make things go well?' royal author Judy Wade previously told PEOPLE magazine.
'But if your sister is a future Queen, she is going to upstage the bride and draw attention. Kate would want to be in the background as much as possible.'
Prince Harry is officially on the guest list for the wedding, although official word has not yet been given as to whether girlfriend Meghan Markle will be joining him on the day.
Pippa and James confirmed their engagement in July 2016 with a statement to the Press Association, which read: 'Miss Pippa Middleton and Mr James Matthews are delighted to announce they became engaged on Sunday 17th July and plan to marry next year.'
In December 2016, HELLO! magazine revealed that the ceremony will take place at St Marks' Church, with a reception being held at the Middleton family home in Bucklebury.
A source told the Sun of the plans for the low-key nuptials: 'She's a traditional girl and just wanted to get married at home.'
'Her mum has been very involved in all the wedding planning and she wants to show off their house to all their friends.'