What a situation to be in!

I think what you do with your SIL now depends on your answers to the following:

1. Does your brother know about your past with his wife?

2. Does your wife know about your past with your SIL?

3. Does your wife fulfill your physical needs?

3. Do you love your wife?

Think about what you have now - it could all be lost in an instant, in pursuit of short term physical gratification.

A last question: Do your wife and SIL have bisexual tendencies? If yes, the perfect solution would be to get them to get into each other, and then you enjoy the thrills of being the third party in the party, who provides the sausage!