It all depends on her will power because we all behave and react to things differently.
There is a lesson to be learned here folks. I know that women like to do this because they are women and that is to take things to their grave. The truth can sometime be your worst enemy since some people can't handle the truth.
He could of played it better by telling them white lies. Everything is the truth but throw in there that it is normal to have fantasy about a celebrity and what not. Cast that same logic onto her as she too may have a thing for what's his handsome face on TV.
I take my chit to the grave because a family affair is the hardest of the burden to bare. It is not her sister's fault for looking so beautiful that her husband is lusting over her. We men, the heck you suppose us to do? Then again, he should of held his ground and don't get too close to her sister. He too is wrong for breaking these undisclosed rules to a relationship. Quit making it awkward than it is.
Now that things are all out there, what to do right? Forgive but never forget. He will forever owe his wife that debt and he can save the marriage by being her slave. Make her happy cuz happy wife is a happy "HER" life.