Hello, I'm sorry to hear this sad news. If your FiL's body has started shutting down I'm afraid that I would expect his death to come very soon.
I'm sorry, but I don't know how travel insurance policies work, but I would expect them to vary from each other. At the most they might only cover your DH's flight and accommodation. Have you got a package deal where the cost of the flights and accommodation and some meals were included? If you have, then the insurance probably covers everything. Is the insurance tied in with the holiday cover, or is it insurance that your bank offers as an extra perk, or insurance that you or your husband arranged privately?
When you know who you got your insurance from you should ring them up as soon as you know when they are next open. If the insurance does cover both/all of you - do you have any children that will be going with you - then yes, I would say do cancel this holiday and book it for later in the year.
If it doesn't cover any of you for one of the two adult's parent dying, then first of all I think it is a very poor show on behalf of the insurance company, but secondly I would say don't actually cancel the insurance until after your FiL has died. If he does die within the next couple of weeks - or will almost certainly die while you are away - then ask your DH to talk to his DM, telling her he is going to cancel your upcoming holiday so that he can support her, and help her with the funeral arrangements. If your DH has any siblings, or his DM has any other support, she may tell you both to go on your holiday, and they will hold the funeral after you return.
However, personally if it was one of my parents who was dying imminently I could not go on the holiday, as I would be far too upset to enjoy my holiday, and would be fretting the whole time about what was happening back home. But every family is different, and has different dynamics, although yours does sound like one of the more caring for each other families.
I think it is most likely that you won't be going on this holiday @wondabar, so I hope that you can recoup at least a portion of it's costs. I will be thinking of you and your DH's family over the next few weeks and I hope that your DFiL's death will be, at least for him, a blessed relief. It was for my DM, which helped quite a lot with my grieving. Good luck OP xxx