Hi,
A few details like the type of pain, area of pain, and how it behaves through the day and with or without activity would help diagnose better.
Have you checked the hydration levels of your sister? It's quite common in monsoon to sweat a lot due to the humidity and the low temperature doesn't make you feel thirsty.. this leads to dehydration... calf muscle cramps are very common in dehydration.
Any history of fall, twist, running, gym workouts, back pain or ankle pain? Any fever? Flu like symptoms? Myalgia (muscle aches) are common after viral flu. Kindly rule out any such possibility.
Next Steps
you can consult me for further assistance!
gentle stretching if pain is due to cramps, and adequate fluid and electrolyte intake is priority