What is the ruling on the elder sister refusing to show her sisters their deceased mother's will, especially since six years have passed since her death and signs of wealth have appeared on her?

It is not permissible to conceal a will, based on the words of Allah the Almighty: "Then whoever alters it after hearing it, the sin thereof is only upon those who alter it. Indeed, Allah is Hearing, Knowing." This includes concealment. If the sister, by concealing the will, causes a delay in the distribution of the inheritance, then the matter must be referred to the Sharia (definition) court.