Sis-in-Law needs a Prayer filled miracle

Since your mother in law is your wife's mom, I wonder if your wife wouldn't need help picking out a new outfit for that date your taking her on ?

It shouldn't be too hard for your wife to come up with a scenario where she need her moms help for a few hours, right ?

As for your sister in law knowing the full extend of God's Grace to her in this situation, I believe that would be her husband's decision

He is ultimately responsible for leading her, not her mom

Personally, I'm not a big fan of the ostrich approach, but that's just me