Does intercourse with a woman whose marriage to the husband would be invalidated by such an act (e.g., the husband's mother or daughter) dissolve the marriage if performed upon the sick husband?
Chapter on Distant Kindred (Dhawu al-Arham)
Al-Mughni
Book of Inheritance Shares (Farā'id)
Primary text
If the sick man engages in intercourse with a woman whose marriage to him would be dissolved by that act, such as his wife's mother or daughter, his marriage to his wife is considered dissolved. Consequently, his wife inherits from him if he dies during the illness, and he does not inherit from her. This applies whether the woman consented or was coerced, as the woman's consent does not constitute an action that invalidates her right to inheritance.
Supporting text
If the husband lacked sound mental capacity (Zawal al-'Aql) during the intercourse, his wife does not inherit anything because there was no sound intention (Qasd Sahih), thus there is no fleeing from her inheritance. Similarly, if the son forced the father's wife while lacking sound mental capacity, she does not inherit on that basis.