It is indeed most disgusting which a person of sound nature would be repulsed even to think.  To cohabit with a woman you have cohabited with her mother.

 

That would also be forbidden based on the principle: that which leads to haram would also be  Haram if you were to marry her daughter what would deter you lusting her mother too?

 

After all, you were involved in an unlawful relationship with her.

 

You are therefore advised to repent of your infatuation with the woman as well as her daughter. For marrying anyone of them, in this case, would be like driving without a break. We are to keep away from all sources of temptation which may lead us to perdition.

 

I would urge you to pray:

 

O Allah, let me love faith and beautify my heart with it; and make me abhor and detest disbelief, immorality, and acts of disobedience.”

 

Almighty Allah knows best.