It may well not have. I've just googled and virtually every result has said that this used to be the case but it no longer is in many/most cases.
And had she had an accident, or been stopped, this is from the AA website
A driver who is involved in an accident or caught without DOC insurance, even if the car they are driving is insured by the owner, will be dealt with severely by the law. They will be heavily fined with up to 8 licence penalty points, and could be banned from driving.
What's more, the owner would also face prosecution for allowing an uninsured person to drive their car. The owner's insurer will have to meet any third party damage and injury costs but would almost certainly cancel the policy. They may then take legal action to recover their costs from the owner, who would also be liable for the cost of repairing their own car.
Insurance companies take convictions for driving without insurance very seriously. Both the owner and the driver will find it very difficult to obtain affordable car insurance in the future. This could be a very good way of losing a friend!
Note all the references to the owner also being penalised.
So OP does not need to unclench. She's discovered that her DH and SIL are not morons, and she's entitled to be annoyed.