Either go all the way and have sex with her or knock it off completely. I'm not for having sex with my sister but if I was in your situation, I bet the idea would be there. You are doing everything she would want in a relationship without sex and honestly, you are getting nothing out of it.
If you sleep with her, there you go and at least you got something out of it. However, if the idea is at all creapy to you, then knock it off and stay with your wife and stop being "intimate" with your sister.
If you sleep with her, there you go and at least you got something out of it. However, if the idea is at all creapy to you, then knock it off and stay with your wife and stop being "intimate" with your sister.