Is it considered an injustice to his daughters from a previous marriage, after his death, for the mother to own the house that the father built on her land, believing it to be in her name? Is the income generated from it considered ill-gotten gain? What should be done to rectify this injustice, and what is the questioner's position if the mother refuses to give his sisters anything?

If the father wrote the house in the mother's name as a will for her to take after his death, this is a will to an heir, which is not permissible in Sharia law unless the other heirs agree. The same ruling applies if it was a gift during his lifetime and the mother did not take possession of it before his death.

However, if the mother took possession of the house during the father's lifetime (by vacating it for her if it was a residential house, or by any form of relinquishment), then this is a valid gift and is not considered injustice. It is permissible for a husband to favor some of his wives with gifts.

In the event that the gift is not valid, the house reverts to the estate and is divided among all heirs. As for the land, it is the mother's property, and the heirs only own the building on it. The mother is a partner with them in it.

The lawfulness of the income generated from the house depends on the above. If it is proven that the sisters have a right in the house, then the mother must be advised to give them their due right.