Complex sentence: Katherine is generally held to have been his youngest child, although, based on her review of the fragmentary evidence, Weir argues Philippa was her junior and that they were children of his second marriage.
Split into simpler sentences: Katherine is generally held to have been his youngest child. Modern-day author Weir argues, based upon her review of limited fragmentary evidence, that Philippa was the junior and that both were children of a second marriage.