Insure one for "any driver". Make sure your no claims bonus is insured too (from what you have told us so far).

After sister arrives, invent emergency that takes you out. Leave her a mobile phone.

Phone while out saying that you are absolutely stuck (even a traffic jam would do), she must go on in the other car and you will catch up in the second.

Catch the train and ring her to say the car you had went wrong (even had a puncture and you could not get the wheel off) and get her to meet you at the nearest station at the far end - thus you minimise the time with her. You still have to get back though.

Ah! you get a discount fare on which a return is cheaper than a single, that way you have to come back by train too.