What is the kinship relationship between the child of the father (husband) and the child of the son (his wife's child from a previous union)?
Chapter on What is Prohibited to Marry and Combining Between Them and Other Matters
Al-Mughni
Book of Marriage
Primary text
If the man and his son each have a child resulting from these respective marriages, the son's child is the maternal uncle (khal) to the father's child. Conversely, the father's child is the paternal uncle (amm) to the son's child.