What are the degrees of affinity (marriage relationship) that establish permanent prohibition (hurmah)?
Chapter on What is Prohibited to Marry and Combining Between Them and Other Matters
Al-Mughni
Book of Marriage
Primary text
When a man contracts a marriage with a woman, she immediately becomes forbidden to his father due to the verse: "And the wives of your sons who are from your loins" (Quran 4:23), as she is among the wives of his sons. She becomes forbidden to his son because of the saying of Allah Almighty: "And do not marry those whom your fathers married from women" (Quran 4:22), as she has been married by his father. She also becomes forbidden to him due to the saying: "And mothers of your wives" (Quran 4:23), as she is included among them. There is no difference of opinion on these points. The grandfather is treated like the father in this regard, and the grandson is treated like the son. This applies whether the relative is distant or close, an heir or not, related through the father or the mother, or through the sons or the daughters of either.
Supporting text
The daughter of a man's wife is forbidden to him, as is her mother, and the grandmother, however high up, is considered like the father, and the son's son, however low down, is considered like the son.