In Dragonfly in Amber, Alexander Randall's dying wish was for Mary Hawkins to marry his brother Jonathan so their child would be a legitimate Hawkins. Why didn't Alexander marry Mary himself, which would have accomplished the same goal, albeit leaving her a widow? They would have had to find someone to perform the ceremony, but that shouldn't have been difficult. Jonathan was willing to provide for Mary and the child in either case.
(I understand that the marriage made the genealogy work out, but I am looking for an in-world reason.)