Next of kin has no legal status per se.
The husband's estate will pass either under the terms of his will (if there was one) or the intestacy rules (ie no will). Assuming the husband predeceased the wife by more than 28 days, the wife will have inherited the entirety of his estate under the intestacy rules.
The wife's estate will either pass under the terms of her will or if no will under the intestacy rules. The husband's brother will not be entitled under the intestacy rules. Assuming your mother was the only sibling of the deceased (and the deceased left no issue or parents) then she is entitled to the estate. However, if there were other siblings who had children who survive, they will also share in the estate.