'My Sister's Husband Is Having Affairs With Me' - Should I Tell Her?

After the completion of my Nysc, I moved to Lagos to stay with my elder sister who told me there are lots of Job opportunities for accounting graduates in Lagos. My Sister is married, with two kids she has a lovely home, she teaches in a prestigious school her husband a Pediatrician and has his own clinic, who promised to help me get a good job to start with, from one of his friends.

When ever they go to work, and the kids for school am usually left alone at home, I spend most of the time, either watching tv or just sleeping.

On a particular day, after i was done with some chores, somebody came into the compound, as i looked it was my sister's husband, I quickly rushed to open the door, forgetting I was putting on a very revealing bum-short, upon coming into the house he gave me a lot of compliments telling me I was a much prettier version of my sister, he told me he forgot some documents, and while he was upstairs he called for me to join him find them in their bedroom. So I went there and was opening and checking cupboards as he was, then all of a sudden someone grab me by the waist and began to say how good looking I am more than my sister and how curvy I look that afternoon, I tried to resist him but it seem as if I was wasting my time, them eventually he had his way with me.

After much apology he left the house.

When my sister came back I felt like telling her the whole ordeal but I was scared it might scatter her home, I felt sad and used up, no justice.

 When the husband came back in the evening he tried to make amends by buying me gifts, and promised to take me out during the weekend.

 When it was weekend I reluctantly didn't want to go out but my sister insisted saying I have been in the house for too long that I needed to have fun. So we went out, he met some few friends and then we headed for a popular eatery around. While we sat, he began to apologise for taking advantage of me, at first I was getting it, until eventually, he said I have always liked you, your sister is not fun at all, in fact she rarely let me touch her this days and even though she does, she lie there like a cold fish, there is no flavour in that, he said, I want us to be friends lets continue like this, I mean you could ask for anything I would give you, I would lavish gifts on you, let's be lovers he said.

At that point I was confused I didn't know what to say, I have never dated a married man, how much more my sister's husband I can never do this to her and I don't want her marriage to brake up. Am confused, I don't know if I should tell her all this things or keep quiet while her husband use me.