Complex sentence: In 1773 she became the second wife of Richard Lovell Edgeworth (1744--1817), a member of the Lunar Society, who corresponded regularly with Seward, although Seward was opposed to the marriage.
Split into simpler sentences: In 1773 she became the second wife of Richard Edgeworth. Edgeworth, a member of the Lunar Society, had corresponded regularly with Seward, although Seward was opposed to the marriage.