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 is His slave and Messenger.
What your father in-law [father of your wife] had left after his death is inheritance for all the heirs, and it is not permissible for anyone to dispose of it as he pleases, by selling it, gifting it or else, except if it is his own share of inheritance. The mother is not permitted to sell the house, and the like, without the consent of other heirs. If the heirs agree and the selling took place, she is not permitted to take the remaining amount of money after having bought another house unless that amount equals or is below her share of inheritance, and whatever she takes will be considered as part of her share in inheritance.
The share of inheritance of the wife from her deceased husband is one-eighth portion, if her husband left children, as Allaah Says (what means): {But if they have a child, for you is one fourth of what they leave, after any bequest they [may have] made or debt.}[Quran 4:12]. For instance, if the house costs six million or that it was sold for this amount, then the wife is entitled to one-eighth of it, which is 750,000.
Finally, it should be noted that in such cases, we advise you to solve the problem peacefully if possible, otherwise, the matter should be taken to an Islamic court if any or to an Islamic centre to study the matter and determine the share of each heir and give everyone his right.