Praise be to Allah.
The wife’s maternal aunt and paternal and are not regarded as mahrams for the husband; rather they are like any other non-mahrams, so he should not be alone with them or shake hands with them.
The fact that there is a prohibition on being married to a woman and her paternal aunt at the same time, or being married to a woman and her maternal aunt at the same time, as it was narrated by al-Bukhaari (5109) and Muslim (1408) from Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (blessings and peace of Allah be upon him) said: “One should not be married to a woman and her paternal aunt, or a woman and her maternal aunt, at the same time,”
does not imply that they are mahrams, because this prohibition only means that a woman and her paternal aunt or maternal aunt should not be married to the same man at the same time; it does not mean that it is prohibited to marry either of them. If the wife leaves her husband through divorce or death, it becomes permissible for her ex-husband to marry her paternal aunt or maternal aunt after that.
It is well known that being heedless with regard to such matters may lead to going against the shar‘i purpose in prohibiting being married to a woman and her paternal aunt or maternal aunt at the same time. Perhaps being careless about mixing and the like may lead to the man or woman becoming attracted to one another, so he may divorce his wife in order to be able to marry her paternal aunt or maternal aunt, or her brother’s daughter, and so on. That in turn will lead to severing of ties of kinship, and will stir up rancour and grudges.
The Permanent Committee for Ifta’ was asked: I married a girl, and she has a paternal aunt (the sister of my wife’s father). Is it permissible for me to look at her and greet her with salaam? Is that permissible or not?
They replied: It is not permissible for your wife’s paternal aunt to uncover her face in front of you or for you to shake hands with her, because you are not one of her mahrams. And Allah is a source of strength." (Fataawa al-Lajnah ad-Daa’imah – vol. 1 17/436).
And Allah knows best.