Complex sentence: After Sly faked his amnesia, he had gone into retirement and formed a relationship with Carmelita, who enjoyed her time with Sly so much so that she was kept oblivious to his coming relapse back into thievery.
Split into simpler sentences: After Sly faked his amnesia, he wanted to retire and forms a relationship with Carmelita. Carmelita enjoys her relationship with Sly so much so that she doesn't notice his coming relapse back into thievery.