My Wifes Neice

Earlier this year my wife asked me if I'd fuck her neice, given the chance to do so.
My wife already knew Katie her nineteen year old neice had the hots for me, and she also knew it had gotten to the point of Katie flashing me her breasts and pussy.
I'd stayed faithful to my wife, but I have to admit it was difficult not to rip Katie's knickers off and fuck her.
Katie will be staying with us again soon, and she's already begun the teasing, by sending me a video of herself masturbating and a message which says "***** (my wife) has told me, she's going to allow you to have me. Can't wait".
I've spoken to my wife, and although she was coy about me and Katie having sex, she wasn't exactly saying no.
Now I'm thinking my wife wants me to fuck her neice, but doesn't necessarily want to give outright permission.
I'm confused, sexually charged and wary all at the same time.
My wife knows I have a large libido, yet I don't want to wreck our marriage, even if Katie is a stunner.
Why can't women just say what they mean and want.