Inheritance shares are the rights that are determined by Allah. One cannot trample others’ rights even parents or siblings. This is a deadly sin.
Inheritance of your father-in-law should be divided into eight. One share should be given to his wife (your mother-in-law), one share to his daughter (your wife), and two shares to each of the sons.
Let your wife ask for her right that is determined by Allah from her brother and mother without any hesitation.
Those who have been left inheritance should consider the following verses, divide the inherited property accordingly, distribute it to the owners, and no one should violate others’ rights.
“Allah gives you the task (of inheritance) concerning your children: one share of the male will be equal to two shares of the female. If there are more than two daughters, they shall split two-thirds of the inheritance evenly. If there is only one daughter, she shall have half of the inheritance. If the deceased has any offspring, each of his parents shall have a sixth of the inheritance. If the deceased has no child and his parents alone inherit him, then one-third shall go to his mother. If the deceased has siblings, then one-sixth shall go to his mother. All these shares are to be distributed after any task given from it (as a payment) or any written debt is paid. You do not know which of them, your parents or your children, are more beneficial to you. The shares are determined by Allah, for He is All-Knowing, and He judges rightly. To you belongs half of whatever has been left behind by your wives if they die childless. If they have (at least one) child then to you belongs a fourth of what they have left behind, after any task given from it (as a payment) or any written debt is paid. To them belongs a fourth of what you leave behind if you die childless. If you have (at least one) child then to them belongs one-eighth of what you have left behind, after any task given from it (as a payment) or any written debt is paid. If a man or a woman has no surviving heirs such as parents or children but has a half-brother or a half-sister from the mother side, then each of these shall inherit one-sixth. If they are more than two, then they shall split one-third of the inheritance evenly, after any task given from it (as a payment) or any written debt is paid, without incurring a detriment. This is a task (given to you) from Allah. Allah is All-Knowing, the Indulgent. These are bounds set by Allah. Allah will let the ones who obey Allah and His Messenger, enter the Gardens in which rivers flow. There, they will settle as immortal. That is the great triumph. Whoever disobeys Allah and His Messenger and transgresses the bounds set by Him, Allah will cause him to enter a fire, and there, he will settle as immortal. He will have a humiliating torment.” (An-Nisa/The Woman 4:11-14)