There are many good reasons to have an affair with one's wife's sister. One is that if they are close and they value family appearances she has a lot to lose and will therefore be ultra-secretive and won't show up on your door confronting you in front of your wife. Additionally, you can be together without attracting suspicion because no one would think it is weird that you were watching TV together and babysitting. The perfect affair.
What happened in my case with one of them is that we fell in love and the guilt was overwhelming for her and for me, but the sex was incredible.
Things went bad for us, but I would repeat it if I had the chance. Bizarre Love Triangle.
If it is your brother's wife, that is an entirely different story. Dicking your brother over is surely against the rules unless you hate him.
I highly recommend affairs with sisters-in-law.