My father had two sons and a daughter and passed away. While alive, he gave my brother his share, which is a plot of land just besides his house. My brother constructed his house there. He told us to divide his house equally between my sister and myself (son) after his death. I have a few questions:
1. Can parents make a will regarding their heirs; like in this case, to divide the property equally between my sister and myself?
2. If we both accept and do it according to what he mentioned, are we not committing a sin ourselves and does the sin not fall on my father too in relation to what is prescribed by the sharia, simply by assuming that we can overwrite what has been prescribed by Allaah?
3. If my sister and myself agree to share it equally, then how should we do it? Should we do it according to what has been prescribed and then I give out a portion of my share to her as a gift to make it equal after having taken possession of the share as prescribed by sharia?
4. Should we assume that it is only by means of a gift that this can be accomplished?

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger.

The answer to your question is summarized in the following points:

1) The plot of land that your father gave to your brother before his death is not considered your brother's share in the estate; rather, it is a gift. It is not part of the deceased's estate.

2) If your father has given this plot to your brother as a gift while he was not ill with a fatal illness (i.e. that is likely to end in death), and your brother took possession of it, then it is a valid gift. However, it is impermissible for the father to give a gift to some of his children, males or females, and deny the others. Rather, it is an obligation on him to observe equality and justice among his children in terms of offering gifts. If the father passed away before making equal gifts to all his children (by taking it back or offering similar gifts to his other children), then the gift is established according to the view of the four Imaams.

Ibn Qudaamah wrote:

“If a parent favored one of his children with a gift and denied his other children or offered one of them better gifts and then passed away before setting right his mistake by taking back the gift or offering the others equal gifts, then the gift offered to one of the children is valid and cannot be revoked or canceled by any of the rightful heirs. This is the stated view of (Imaam) Ahmad ... (Imaams) Maalik, Ash-Shaafiʻi, the Hanafi scholars and most of the scholars. Another view of (Imaam) Ahmad, on the other hand, held that the heirs are entitled to revoke and cancel that unfair gift... This view was adopted by ʻUrwah ibn Az-Zubayr and Is-haaq. (Imaam) Ahmad said, 'ʻUrwah narrated the three ahaadeeth of ʻAa'ishah, ʻUmar, and ʻUthmaan in this regard. However, he relied, instead, on the Prophet's hadeeth that reads, 'It is to be taken back, regardless of whether that be during the gifter's lifetime or after his death...''” [Al-Mughni]

The view that the unfair gift should be taken back even after the death of the gifter was chosen by Shaykhul-Islam Ibn Taymiyyah . He said:

“He must take the gift back in his lifetime, as the Prophet, sallallaahu ʻalayhi wa sallam, commanded. If he passed away before taking it back, then it should still be taken back according to the most correct of the two views reported in this regard, in obedience to Allaah and His Messenger and in compliance with equality which sharee'ah commanded us to observe among one's children in terms of gift-giving and following the example of Abu Bakr and ʻUmar in this regard. It is impermissible for the son who was favored over his siblings with this gift to take it; rather, he has to share all property with his siblings according to the just laws of Allaah.” [Majmoo' Al-Fataawa]

3) If we adopt the view that such a gift is valid and binding after the gifter's death, then the son who received the gift is still entitled to share the house which the father asked to be equally divided between his brother and sister with the two of them. The house is not considered the property of the brother and sister only just because the father gave the other son the plot of land as a gift. Rather, the three children have shares in the house according to the sharee'ah.

4) The will made by the father saying that the estate or part of it is to be divided equally between the son and daughter is non-binding to begin with because it contradicts the command of Allaah. Allaah, The Exalted, says (what means): {Allaah instructs you concerning your children: for the male, what is equal to the share of two females...} [Quran 4:11] Please, refer to fataawa 116588 and 24693.

The will to divide the house equally gives the daughter more than her legitimate share and involves giving an heir a part of the estate as a will. Since there is a third son who is entitled to inherit, the house cannot be divided equally between you and your sister even if you consent to his will unless your brother gave up his right to his share in the house and sufficed with what his father gave him as a gift.

Finally, we draw your attention to the fact that the division of inheritance is a serious and complex matter. It is not sufficient to settle for a fatwa issued based on the information provided by the questioner. Instead, you should refer the case to an Islamic court for investigation. There may be other heirs who might not be discovered without careful investigation. Moreover, the deceased may have left another will or been liable to debts or other obligations and liabilities that the heirs are not aware of. It is well-known that the settlement of debts and liabilities is given precedence to the heirs' rights to their shares of the estate. Therefore, you should not divide the estate without referring the case to an Islamic court, if available, in order to secure the interests of both the living and the dead.

Allaah knows best.