Why does he continue his affair?

I fought with my husband for a year before he admitted cheating with his cousin while he was in his home country for a few months. He gave me the story it had nothing to do with love it was just sticking his **** in a hole and other crude stuff. He said I wasn't supposed to find out and he didn't want a divorce. He doesn't care I hurt. Well a few months ago we were fighting and he told me he can't live without me and he wants to grow old with me, blah blah. But yesterday I saw a facebook conversation from him to that cousin's sister. he was writing how he couldn't sleep or work. He missed his cousin and he wants her. But he doesn't want to be second option to her stupid boyfriend. And he was trying to get his cousins sister to call her but the girl didn't want to answer the phone and talk to him because she said he left her to go back to his family (me and the kids) and she doesn't want to start an affair again. But I thought me and my husband were ok and trying to work things out. But the message also said he had been talking to her for the past few weeks and thought everything was ok and then she seemed distant and then stopped talking to him altogether. So I assume he has been sneaking off somewhere or at work to talk to her again. When I asked him who he was chatting with he was laughing and saying my cousin. I asked him if they were talking about the girl he had the affair again and he said no just his problems and none of my business. So I snooped and then when he got home that night he changed his password......but it was too late I already saw everything. I don't understand why he can spout things like "I can't live without you" to me and then be so obviously in love with his cousin. I never believed they just had sex a few times because it "just happened" because he came back from his country a different man ...all depressed and stuff. I am planning to divorce him that's not the question. How can he have sex with me here all these months but have been pining for her. What the heck is that? I wonder if he has psychiatric problems because this seems to extreme to just be a cheater and a88hole. Does anyone know any legal options I have for keeping him away from our kids? I am concerned because his affair was INCEST! Also his message says he was trying to get her to meet him in Mexico in July and then sneak her across the border. How can I report this or get them in trouble for this? It really scares me how he can be so normal with me at home and have all this stuff going on in secret..............