A few things:
1. Your sister-in-law's husband sounds like a complete asshole.
2. Your sister-in-law may not have been very smart in marrying him, but I'm sure not one to talk.
3. Cheating due to a lack of sexual variety is, well... I don't know what it is, but it's pretty damn stupid.
4. This is ultimately none of your business except in terms of supporting your wife, and possibly your sister-in-law if she wises up and decides to leave this jerk.
5. Prison is not a pleasant place to be.