Is it permissible for my grandmother to distribute her house two years before her death, such that her three sons receive three-quarters of the house's value, while her five daughters receive the remaining quarter?

If the grandmother gifted the house to her children and they took possession of it before her death, then the gift is valid, but she is sinful for not being just. They should return the gift and divide it justly.

However, if the house was her residence and she did not move out of it until she died, then the gift is void due to the lack of possession and taking hold of it before death. Mere writing is insufficient, and a gift is not complete without taking possession.

If she conditioned their disposal of it after her death, then it is a will. A will to an heir is invalid unless the heirs agree, based on the Prophet's (peace be upon him) saying: "Indeed, Allah has given every rightful person his right, so there is no will for an heir unless the heirs wish." In this case, the house is considered an inheritance to be divided among the heirs according to Islamic law.