Tricky one.....You could try your home insurance but not 100% sure on that one. You're cutting things fine really due to the time element.
You could try contacting both parties involved and request a change of date ie move it to next year - but both will incur charges for changes, not only for the move but also because of price differences.
Another option would be to pass the holiday on to friends - again this will incur amendment fee's.
We always buy annual insurance as we travel a few times a year - and we always know we are covered - obviously ok if you are like us, but either way you should always ensure you book travel insurance at the same time as you book a holiday. Too late I know but a lesson learnt.
If none of the above are an option and it looks like you are going to have to 'give up' the holiday - then leave it until the last minute. Things may change with your father in law and you may still be able to go.
Hope someone can come up with something more positive.
Thinking of you and your family.