Complex sentence: The passage of the Fugitive Slave Act of 1850 increased the danger to Bibb and his second wife Mary Miles, because it required Northerners to cooperate in the capture of escaped slaves.
Split into simpler sentences: The passage of the Fugitive Slave Act of 1850 increased the danger to Bibb and his second wife Mary Miles. It required Northerners to cooperate in the capture of escaped slaves.