Is it permissible for the father and son to claim shares of their deceased mother’s house, even though they had previously relinquished their shares in it in exchange for the daughters relinquishing their shares in the father’s farm? And is the son entitled to take his share of the house on the pretext that he undertook to prepare a place for investment there, and to give his share to his father, knowing that the daughters did not receive a percentage of the investment’s value, and they refuse to sell the old house?

If the intention behind the daughters' shares in the farm was their portion of the inheritance, then the waiver is void, because it is not permissible to distribute an inheritance during the lifetime of the inheritor, and the farm remains the property of the father. The right of the heirs to the deceased's estate remains until the estate is divided. As for what the father and son spent on the house with the intention of reimbursement, they are entitled to reclaim it from the estate. If it was spent as a voluntary act, they are not entitled to reimbursement.

Regarding the revenue from the investment, there is a difference of opinion. Some scholars believe that it follows the principal and is divided among all heirs. Others believe that it belongs to your husband and his father. Yet others believe it should be divided into two halves: one for your husband and his father, and the other for the remaining heirs.

If their shares were from the ownership of the farm through the father gifting it to them, then the preponderant opinion is the obligation of equitable distribution among children in a gift, and if equity is not achieved, the gift is void.

Your husband must return to his sisters their share of the investment revenue, advise his father, and repent to Allah by restoring rights to their rightful owners.