Complex sentence: Clotaire may have married his sister-in-law out of pity of a girl who was not deemed marriable due to her lameness or possibly as the death rate in childbirth was high, Aregund succeeded her sister to foster her orphaned nephews and nieces.
Split into simpler sentences: Clotaire may have married his sister-in-law out of pity, as she was not deemed marriageable due to her lameness. Alternatively, as the death rate from childbirth was high, Aregund may have succeeded her sister to foster her orphaned nephews and nieces.