You can name who like as next of kin but they have no legal powers, rights or responsibilities.
Intestancy laws for married partners and civil partners state that partners who separated informally can still inherit under the rules of intestacy.
If there are surviving children, grandchildren or great grandchildren of the person who died and the estate is valued at more than £270,000, the partner will inherit:
all the personal property and belongings of the person who has died, and
the first £270,000 of the estate, and
half of the remaining estate.
If there are no surviving children, grandchildren or great-grandchildren, the partner will inherit:
all the personal property and belongings of the person who has died and
the whole of the estate with interest from the date of death.
Parents, brothers and sisters and nieces and nephews of the intestate person may inherit under the rules of intestacy. This will depend on a number of circumstances:
whether there is a surviving married or civil partner
whether there are children, grandchildren or great grandchildren.
in the case of nephews and nieces, whether the parent directly related to the person who has died is also dead
the amount of the estate.
From what you have written, I assume that your brother and his wife did not have children and so my understanding is that everything will go to his wife.
www.citizensadvice.org.uk/family/death-and-wills/who-can-inherit-if-there-is-no-will-the-rules-of-intestacy