How would you feel about looking your brother in the eye after that
You're an idiot and a sleazeball.
It could be the wife's sister as well as his brother's wife ... both sisters in law.
I understand that a sister-in-law can be very sexually alluring to a man for a lot of reasons. For one thing, take note that the wifes sister might have some of the appealing qualities seen in her older or younger sister which attracted him to her. BUT ... it's a very dangerous attraction in terms of the harm that can be done in having any romantic relationship with a sister in law. It can destroy a your marriage or destroy forever your relationship with your brother, whichever is the case.
It has happened more than a few times. But I urge you to do whatever it takes to get the notion our of your mind and to do whatever it takes not to have or create any kinds of "opportunity" to pursue anything (i.e. don't be alone together or anything like that).
CONCLUSION: The odds are very high that you will come to regret it, if you act on that sexual attraction and so will she. It's about the worst scenario of cheating you can come up with (other than something involving incest).
Other people asked questions on various topics, and are still waiting for answer. Would be great if you can take a sec and answer them