My impression is Mrs Craven invited Frank's wife, Mary to join her on holiday, probably on the Isle of Wright - and Jane's mother may also go... 'I have been kindly pressed to stay longer here, in consequence of an offer of Henry's to take me back... in September.' Again, Jane is dependent on a brother for travel, but can stay at Godmersham a few months.
She may, however, have preferred to be home in the July.