In another thread in the GR we discussed Kitty and Mary leaving Jane to burden all the responsibilities and that it seemed to change after the arrival of Lizzy and Mrs Gardiner. Kitty adopting her elder sisters view of Lydia looks plausible to me. As Nasima said Kittys comment about the elopement is made early, just after their return from Lambton. Kitty is impressionable and after Lizzy and the Gardiners returned she must have been exposed to their disapproval of Lydia.