Complex sentence: He then married Cersei Lannister to ensure House Lannister's support for his rule, but remained attached to the memory of Lyanna, which estranged Cersei, and was unaware that Cersei's three children were therefore fathered by her brother Jaime.
Split into simpler sentences: He then married Cersei Lannister to ensure House Lannister's support for his rule, but remained attached to the memory of Lyanna, which estranged Cersei. He was unaware that Cersei's three children were therefore fathered by her brother Jaime.