Spirit Airlines flight 2045 encountered turbulence during descent into Harry Reid International Airport (LAS), Las Vegas, Nevada, causing injury to two flight attendants (FAs). The encounter occurred while the FAs were preparing the cabin for landing. According to the flight crew, while flying the CHOWW4 arrival into LAS, they observed cumulous clouds along their flight path but did not observe any returns on the weather radar, nor on their electronic flight bag (EFB) weather application. In addition, there were no pilot reports (PIREPS) for the area. Due to the cumulous clouds in the distance, they decided it would be safer to seat the flight attendants and passengers early. At approximately 22,000 feet, the flight crew turned ON the fasten seat belt sign and the captain communicated to the lead FA they might encounter turbulence. The lead FA informed the flight crew that they had already conducted passenger briefings and were seated. While descending through 17,000 feet, the airplane penetrated a cloud and encountered approximately 3 seconds of turbulence. The flight crew reported the turbulence to air traffic control and requested to slow the airplane. At that time, the flight crew was informed that a FA in the aft cabin was injured. The flight crew coordinated with airport operations to have emergency medical personnel meet the flight at the gate. The flight continued to LAS and landed without further incident. According to the FAs, they had completed trash pickup and the injured FA elected to use the lavatory prior to initial descent. While in the aft lavatory, the FA heard an announcement from the flight crew but did not hear the announcement clearly. While standing in the lavatory, the airplane encountered turbulence and the FA fell on the floor, injured her leg, and was unable to stand. The injured FA was assisted to their jumpseat and evaluated by an onboard medical professional. The medical professional sat on the floor in the galley near the injured FA during landing. After arriving at the gate, the injured FA was transported to the hospital and was diagnosed with a fractured ankle bone. One additional FA fell during the event and was transported to the hospital; they were released with minor injuries. No other injuries were reported and there was no damage to the airplane.