Opening up your wife to the idea of poly sounds hard enough. Opening up the concept of poly to family members is also very hard more often than not. Add into it the somewhat
incestuous aspect of two sisters sharing one man and you're looking at a vey tough challenge. Not only will your wife have to deal with you having sex with another woman, putting your penis in another vagina/your mouth on it, but she will have to deal with the image of it being her very own sister. For the sake of your wife's relationship with her sister, I would be very careful even broaching this.