I was about to write something similar to amonkie. In your case you are certainly deprived of something. You cant change your wife. Also you cant mess the relationships. If you think the closeness with your SIL might be risky, shift the attention. Sure your SIL will understand and that may help her to find alternatives as well.
The fact she is your SIL itself could be a reason for the closeness. In the beginning the trust and approachability must have paved way for a good start. Now it all can go totally upside down if taken lightly.
You sound very responsible. You will certainly cross this tough time. Good Luck.
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http://www.tfproject.org/tfp/tilted-...ister-law.html