We’re forbidden to marry four classes of women we’re related to by ‘Sihriyyaf (relatives of our partner which, become our relatives at marriage). These classes, Hurmat-i Musaaharah, are as follows: Mother-in-laws: Regardless of the partners having been in zifaaf or not, the son in law is haram for the brides mother from the moment of the nikaah.

Step mothers: The partners having been in zifaaf or not, the wives are haraam for the groom’s father and grandfather. Step daughters: Daughters and granddaughters of the women from her old partner is again haram. However, for it to be haram, the husband must have been in zifaaf with his wife or have touched her with passion. If on the other hand, the wife passes away or the couple split up before this happens, it is then not haram to do nikah with, (marry) the step-daughter.

Even if it means that the man has been in halvat-i sahiha with his wife. (Halvat-i Sahiha occurs when a man and woman are alone, without the disturbance of a third person and when they are somewhere that no-one else can see them.
Brides (daughter-in-laws/ granddaughter-in-laws): It is haram for a man to marry his son or his grandson’s wife, (it doesn’t matter if they’ve been in zifaaf or not).