The policy clearly states that if you or anyone in your immediate family has a pre-existing condition.
This is from my current Allianz travel insurance policy:
"You aren’t covered for any loss that results directly or indirectly from any of the following general
exclusions, unless they’re included in Section 2,
What this policy includes.
The following things if they affect you, a traveling companion or a family member, whether the family member is traveling with you or not:Unless you have an existing medical condition (unless you have existing medical condition coverage)"
An immediate family member who's illness causes you to miss the trip would fall under the "pre-existing" conditions clause, if you knew about this condition at the time of booking or they had been treated for that conditions within the previous six months.
If you or immediate family (mother, father, brother, sister, child or sibling) have a medical issue that may impact your trip it is ALWAYS wise to purchase the pre-existing condition waiver. You usually have to purchase the insurance within 7 to 14 days of initial deposit or initial payment of your trip.
Sorry this happened but that's the policy and it really isn't in "fine print".
I hope things go well with your father.