What is the ruling on the prayer of one whose menstruation has ceased, but she did not perform ghusl until two days later due to her inability to do so?

Allah made earth a purifying agent that takes the place of water when water is unavailable or cannot be used. The evidence for this is the Almighty's saying: "And if you find no water, then perform tayammum with clean earth."

And the Prophet, peace and blessings be upon him, said: "Indeed, clean earth is the purification of the Muslim, even if he does not find water for ten years."

Therefore, your two sisters must perform tayammum and pray when their menstruation ceases if they do not find water. If they did not do so, they must make up the missed prayers.