It is correct that the general rule under Muslim personal laws in India (Shariat) is that a Muslim may, by his will, dispose only up to one-third of his property which is left after payment of funeral expenses and debts without the consent of his heirs.
The remaining two-thirds of the testatorís property must go to those who are his heirs at the time of his death. With respect to Shia Muslims, the bequest of the permissible one-third may be made either to an heir or to a non-heir.
However, with respect to a Sunni Muslim, the one-third bequest may be made only in favour of a non-heir without the consent of his heirs. If it is made to an heir, it is invalid unless consent of heirs is obtained after death of testator, in which case it is valid.
However, the above restriction on disposition of property by will does not apply to a Muslim whose marriage is solemnized under the provisions of the Special Marriage Act, 1954 (instead of Muslim personal laws). This is because upon solemnization of marriage under this Act, the rules of Muslim personal law cease to apply to him with respect to matters of succession of his property. Accordingly, such a Muslim may, by his will, dispose his entire property without the requirement of his heirsí consent.
It would also be open to a Muslim to transfer his entire property during his lifetime by way of a gift to any person, unless the gift is a marz ul maut (death bed) gift. Marz ul maut is regarded under Muslim laws to be a malady which induces an apprehension of death in the person suffering from it and which eventually results in his death. A gift made by a person suffering from marz ul maut cannot take effect beyond one-third of the estate of such donor after payment of funeral expenses and debt, unless the heirs of the donor give their consent after the death of the donor to the excess taking effect.
So in your case, need to know on what conditions the Will has been claimed to be made(with the consent of other heirs or was it transferred to your sister as a Marz Ul Maut