Yes, factor, I think they probably would have.
The strange thing about many insurance matters like this is that policyholders are often penalised for telling the truth even though there is no significant difference to the risk that the insurers cover. As has been said, if DancingDoris’s sister had simply said she drove to her mothers, left the car there and went out there would have been no problem and I doubt the insurers would have looked twice at the claim. By stating precisely what happened, even though it made no appreciable difference to whether the car would have been stolen or not, the insurers repudiate the claim. And insurers wonder why their policyholders tell them porkies from time to time.
I understand them denying liability in instances where the risk is greater (if, for example, she said the car was normally garaged at night but it was always kept on the highway). But in this case the person who last drove it and (nominally) has it in their “custody and control” does not alter the risk.
I would be interested in the outcome, but I fear the insurers will stick to the “small print” in their policy and the arbitrators will have no choice but to rule in their favour.