If the mother deeded the house to the four sons and did not remove the son's name when he passed away then the 1/4 of the house still belongs to that deceased son. It then passes through to his estate and if the beneficiary of his estate is his wife then yes she is entitled to 1/4 of the proceeds.
In reality at the time of the death of her son, the deed should have been changed from your brother's name to his wife's name or some provision should have been made for the daughter-in-law to disclaim it.
If no provision was made at the time of the son's death to remove his name from the deed you are going to have to deal with her being entitled to her 25%.
You can try to reason with her and see if she will voluntarily give up her ownership or possibly accept a smaller settlement than the 25%.
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