What about your wife???
just say **** it and tell her.... i myself am head over heels in love with my mil, she is my heart, my love and all my passions, i dream non stop about ravishing this woman, she actually caught me in her closet one day wearing one of her dresses and literally having intimate sex with her pretty shoes, God i love that woman, she is beautiful, and sweet, and looks so sexy dressed up, she is just absolutley gorgeous, i love the way she walks, talks, smiles, and oh yeah, the way she carries herself and the seductive ways she sits, crosses her legs, or plays with her heels.... her high heels are my sex toys, i dont care who knows it....
Really, Really. You don't, you would be dishonoring yourself, your wife, and your mother-in-law. Surely, you don't expect her to say yes, and not tell everyone? This is one fantasy that should be kept in the la-la land.
Don't do it? In the long run it will create more nasty problems than you may want to face. Just because you want it does not necessarily mean it is good for you.