I take it she was not impressed. You really do need to explain to Mrs Nick that the rest of the country operates to British Summer Time, not, as she seems to think, British Sometime.
Even if you left right now you would not be in Winchester much before 9pm by whichever route you travel.
In your shoes I think I might well have told my sister that we would now be arriving tomorrow then sit back and wait for Mrs Nick to announce her readiness to depart before telling her that the car, with you and the Boy, will be leaving after breakfast in the morning whether she is in it or not.