If you yourself are renting a car, then you would have to check to see if your own personal automobile insurance policy would cover you for either liability protection, Loss and/or Damage or hopefully both.
For example, assuming your two daughters were signing on to YOUR rental contract as authorized additional drivers, they would automatically be covered by whatever insurance you have/purchase (provided all drivers confirm to all terms and conditions of the rental agreement, sign on to the rental contract and meet the minimum age criteria, credit identification requirements and drivers license requirements)... they do not need to provide nor purchase the Rental Operator's insurance options.
If one of your daughters will be the one to rent, then they can check for possible coverage through either their own personal insurance, coverage through a credit card, or optional Rental Operator car insurance options that may be purchased if needed or wanted.
The Rental Operators provide no liability protection inclusive in the rental rate. The renter would either need to use their own personal automobile insurance coverage to cover liability during your rental, or purchase liability coverage from the Rental Operator.
There are a number of possible insurance options that will be provided by the Rental Operator, available to whomever is actually renting the car:
Additional liability protection, which becomes primary coverage if purchased. This is only a required purchase for a renter who has no personal automobile insurance coverage of their own.
Pricing and availability for various car insurance options will vary by location and are made available at the time the rental car is picked up.