I don't really understand your question. If you take out a family policy, then the family should be covered, but please check before you travel. If you take out a policy in one family member's name then I suspect only that person is covered.
If you have specific requirements about an individual family member who might have a pre-existing health problem, then you'll need to speak to a specialist insurer or a broker.
My son broke his leg a couple of weeks before we were due to travel. We cancelled our trip, and the insurance covered the loss (deposits, flight costs etc) for ALL the family.
My son broke his leg a couple of weeks before we were due to travel. We cancelled our trip, and the insurance covered the loss (deposits, flight costs etc) for ALL the family.