Is the husband's will, written before the birth of his children, which divides the property equally between him and his sister, with preference given to his sister for the house with the higher income, considered unjust to his minor children?

The question is vague, but what is understood from it is that your husband and his sister inherited two houses from their father. Then, the husband bequeathed one of the houses to his sister and the other to himself. If it is confirmed that the will is for the deceased, then it is enforceable unless he retracted it. If the only heirs are the husband and the sister, then the husband gets two-thirds of the two houses, and the sister gets one-third. If the husband bequeathed one house from his share to his sister, this means he bequeathed a part of his share to her. It should be examined whether this part is equal to one-third of the estate or less. If it is less than one-third, the sister takes it even if the heirs do not approve. If it exceeds one-third, the sister only takes up to one-third, and anything exceeding one-third depends on the approval of the adult, sane heirs. Since his children are not adults, their permission is not considered. The matter of inheritances is serious and must be referred to the Sharia (definition) courts for investigation.