If a couple that wanted to get married was related, either by blood or by marriage, they may have needed dispensation. For example, until well into the 1900s, it was illegal to marry your deceased spouse’s sibling. My great-grandfather needed royal dispensation for his marriage to his late wife’s sister. Marrying your first cousin or other close relative also required dispensation. These dispensations can hold key information about the family relationship.
For an example, see My great-grandparents’ incestuous marriage.