I never expected this to happen...?

My husband and I have been making strides in repairing our marriage. The love making is as good as its ever been, but i can tell that He isn't making love to me. His pion for me is gone and I don't think he really desires me. Him and my younger sister are close. He saved her life, and there is a father-daughter/brother-sister relationship between them. On the other hand, she all but hates me, especially for hurting him. She is 21 now (He's known her since she was 14). They write to each other everyday (ever since we've moved and before my affair) The truth is, she is the result of an affair my mother had on my father, and I never treated her right, nor has my mother. He recieved a letter from her in the mail today and i'm freaking out. She is professing her love for him basically. She tells him that I don't deserve him and she tells him about dreams she's had/having about him. How she has thoughts of taking him away from me. How she wants him to be her first it's digusting. plus other things I can't/won't mention. I'm not worried about my husband because he usually stops her before she starts saying things like that to him (out respect for himself and her, he wouldn't do anything. Although he didn't mention he wouldn't do something like that because he loves me.), and it hurts because of what I did. But I can't believe my own sister. Never in a million years. She is coming to see us(him) for the holidays. He doesn't know I've read the letter. And doesn't really hide any of them. I read others, but none are so...foward and revealing. How do I handle this? Will he be mad about me reading the letter? do I have the right to say no to her visiting? She is beautiful and my husband loves her to death. Will she really try something?