It is permissible to make a bequest to an heir or a non-heir in up to one third of the estate without the need to obtain the consent of the heirs. Additionally, it is permissible to bequeath more than one third of one's estate though the excess portion is not executable except after the heirs authorize it. The heirs must be adult, sane, not suspended from dealings, not coerced into consenting and knowledgeable of what they are authorizing.

If some of the heirs authorize the portion in excess of one third and others do not, then this excess is executable only from the portion of the heirs who consent to it. This is the opinion implemented in Egyptian courts and for fatwa. The Egyptian law of inheritance, law no. 71 for the year 1946 CE implements this opinion which is likewise one of the opinions of Islamic law in the matter.

It is not permissible to contest matters over which there is a difference of scholarly opinions. The legal principle states that the ruling laid down by authorities lifts manifest and concealed differences.

Bequests are executed before estate division due to the words of Allah the Almighty Who says,

(The distribution in all cases is) after the payment of legacies and debts. [An-Nisaa`:11[

Based on the above, your mother's bequest is permissible. The heirs do not need to authorize it if the bequeathed land is equal to one third of the estate or less. But as previously mentioned, it is necessary to obtain the consent of the heirs, each concerning his share, if the bequeathed land comprises more than one third of the estate.

Allah the Almighty knows best.