Some terrible advice on here, I used to work in this field.
You must declare that your DD is having investigations atm. Otherwise the insurers may not pay a claim.
The problem you will face is the destination being the US, who are notorious for exorbitant healthcare costs, should DD be admitted to hospital, this is why insurance to the US is so expensive.
Had you been travelling to Europe on a single trip policy the underwriters may not be so concerned.
My advice would be to book with someone like Staysure, but be prepared that they may not cover for the undiagnosed condition, so that would mean, should you need to cancel, curtail, medical treatment that may reasonably be connected to the condition/s for which your DD is being investigated for.
Don’t bother booking a policy for the rest of the family and one for DD later, if you needed to cancel or curtail due to DD undiagnosed condition you will not be able to claim, as insurance is there to cover the unexpected, the fact DD is having investigation negates this point.
The golden rule with travel insurance is to book as soon as you book your trip, that way you are insured for cancellation should this situation arise.
Good luck, I hope DD is ok and you have a good holiday.