Should I go in for an extra-marital affair?

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I am 28 years old and am happily married for three years. I love my wife and we enjoy a good relation with her (both emotional and physical).

However, there is this one niece (age 18 years) from my wife's side who has lately driven me crazy. It all started 2 years back when I used to help her in studies. She would get unusually close to me while I taught her. She would rest her arms in my laps and even brushed her breasts, giving me erections. Again when she walks in street with me, she would get close and intentionally touch her breasts against my arms. Lately, she has also started discussing with me on her sexual talks with her FB bf and that how dirty she has become.

Now, with all this bombardments I have become sexually attracted to her. I love my wife and don't want to hurt her either. But I am helpless, as I am having very high sexual craving for her to an extent that I get erections whenever she comes close to me.

Our final encounter, just three days back, was a very serious one. She asked me to take her to my another apartment, which is empty, as she wanted to have a drink. We both had only two pegs and were not out of senses. Thus, none of us were bold enough to make a move for sex.

Please help me friends to understand what I should do in this situation? Should I go in for a casual sex and be normal with my wife like nothing's happened or should I completely avoid her (which I feel is difficult for me now)?
thank u all fr thrashing, disgracing and insulting me. I really needed someone to discourage me fm doing ths sin. I knw i ws wrong..bt gt carried away. I m a human and m prone to certain weaknesses. Believe me m not really a perv...i just got drowned. Anyways really appreciiate u guys and thanks once again.