Sisterly Love or Plain Insanity?

So here’s the scenario:
Your famous sister(actor,musician whatever), whom you LOVE to death drops in to visit you and your family (husband and kids). That evening during a drinking game, she asks to sleep with your husband. You brush it off even though you know she’s serious, as she was kind enough to say NOT ask, “you don’t mind do you”
The next day she tries to “accompany” your husband to the market; you understandably say hell no and She wanders off screaming and upset. *pause*
I recently saw a movie called Hilary and Jackie; simply put it’s the true story of two sisters who shared a passion, a madness and a man. 

Hilary is the famous Cellist, and Jackie is her loving housewife sister. So Hilary wanders off screaming, and understandably Jackie goes after her seeing as their “close” and all. She finds Hilary naked, crying; having some panic attack. Screaming, “all i wanted was a f*ck”. Feeling like she has no choice, Jackie asks her husband to sleep with Hilary, saying it will prove that she’s loved…He relunctantly agrees…
That scene left my jaw hanging… I have sisters and I love them to death but there is no way in hell I would let any of them/anyone sleep with my husband?!
I know they were close n all but that’s just crazy… then again that’s just my opinion…
So… Would you let your sister sleep with your husband if she said it would prove you truly loved her?