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, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
No doubt, the wife has more right to wash the body of her dead husband than his mother. Moreover, the man’s mother has no right to wash his dead body unless there is no one to do it other than her such as his wife or even a man who is not his relative.
An-Nawawi says in Al-Majmoo', “If the deceased left behind a wife, it will be permissible for her, in our sight, to wash his body. That is the opinion of all scholars except Ahmad according to one of the narrations reported from him.
Whether she should be given priority over his close paternal relatives is open to two different opinions that were mentioned by the compiler, and they are famous. The more valid in the view of the majority is that she should not be given priority over them. The close paternal relatives should be given priority, the kinsmen second, foreign males third, then the wife, and the women he was a Mahram to should be last. This is the statement of the compiler in At-Tanbeeh and Al-Jurjaani in At-Tahreer. The other opinion is that the wife should be given priority over them all and it is authenticated by Al-Bandaneeji .”
Some juristic scholars are of the opinion that each spouse has more right than close paternal relatives to wash the dead body of the other. Commenting on the statement of Khaleel, Al-Kharshi said: “And both spouses should be given priority (to wash the dead body of one another) if their marriage is valid. That is, if one spouse dies, the other should be given priority over all guardians to wash his/her dead body. And, in case of dispute over this issue, the judgment should be given in his/her favor because the basic rule is that the right should be given to whom it is proven.”
Allaah Knows best.