I'm in a sexual relationship with my wife's sister. how do i proceed?

First some high level details:

- I'm 31
- My sister in law is my wife's sister
- Sister in law is 19

I married my high school sweetheart. My wife and I dated for several years and we've been married for almost a decade. 3 years ago our marriage became a family with now 2 children. We are a typical "white picket fence" family. I love my wife.

When my first child was born I started to be around my in laws a lot. The saying "It takes a village to raise a child." is no lie. Also they live less than a mile away. Being around my in laws also means I am also around my sister in law.

My sister in law and I are very similar. It is easy for the two of us to relate and talk. Over 2 years we really got to know each other. As she got older I became attracted to her. I kept this to myself, but naturally anyone that is attracted to someone else flirts in some sort of fashion. I started to notice she would flirt back. Feelings for her started and my attraction to her grew, but again I kept this to myself because she was about to head off to college on the other side of the country.

She spent a year at college, and decided to transfer. Her choice brought her back home. I noticed that the flirting she once did was now more of a desire: sitting close, glancing at my lips, licking her lips, she even made a few attempts to hold my hand nonchalantly. All of this compelled me to tell her about my attraction and feelings for her.

I asked her if I could take her out to lunch because I needed to tell her something. She accepted and the following day I took her to lunch. I explained to her that I was incredibly attracted to her and that I had feelings for her and that I'm telling her this because I'm getting the impression from her that she may have the attraction and feelings I have on some sort of level. A long silence followed, but she eventually responded with, "I do."

Our conversation during lunch didn't end with "I do.", but I don't have the time in the day to type it all out. The conclusion of it ended with, "Let's see where it goes." Well it went and went fast. A quick kiss after lunch turned into sex after 2 weeks. Which really caught me off guard since she was a virgin. I later found out that my impressions of her desire for me started a lot sooner than I assumed. She told me she started fantasizing about me a couple years ago when my wife let her use our home computer. She was looking at our photos and noticed a folder called private. Well it has some XXX rated pictures and video of my wife and I together. My sister in law admitted that she copied a few of them to view later on.

My sister in law and I have both verbally expressed that we are in love with each other. I still love my wife. I love my sister in law and wife on the same level. I never understood plural marriage and "sister-wives" until now. My sister in law and I have talked about what we should do next. She expressed that she is perfectly fine continuing with our relationship for the short term. She has ambitions to be married and have a family after college, but she knows that those ambitions will not be fulfilled with me if our relationship stays as it is. I have a decision to make in the next year or two. My wife or sister in law.

My sister in law expressed to me she would understand if I were to choose my wife, and that her sister's heart wouldn't be broken, her relationship with her sister wouldn't be ruined, and her nephews wouldn't have divorced parents. That our time together would be between us and she would hold no grudges or regrets. She also told me that she isn't going to be totally selfless. She would be ecstatic if I were to pick her and for us to get married and have children.

I know there are a lot of downsides of picking my sister in law, and the only reason why I am considering my sister in law is because she makes me "happier". I'm not unhappy with my wife at all. An example would be: I love chocolate, but I like milk chocolate more than dark chocolate. My sister in law and I have similar likes: movies, food, entertainment, music, travels, etc... Where as my wife and I have dissimilar likes. Even with all of the hard aches and hurdles with picking my sister in law I would be "happier".

So let me hear your thoughts, and let me know who you suggest I pick: wife or sister in law.

BTW... Before you even suggest it. Both is not an option. There is no way in hell I could convince my wife and sister in law to share me.