Complex sentence: Caroline Phelps eventually married James Boulter Stokes, brother of the dead Josiah, and he would became the 3rd son-in-law to join Phelps, Dodge & Co. as a partner.
Split into simpler sentences: Caroline Phelps eventually married James Boulter Stokes, brother of the dead Josiah. He became the third son-in-law of Phelps to join Phelps, Dodge & Co. as a partner.