Question: "Hi, I find myself lusting over my sister-in-law (wife’s younger sister). B”h this is not my main struggle, however it does come and go occasionally. I think it’s because she looks more feminine than my wife. She got married recently and she is very attractive and always put together. Any tips?"
Reply:
It can be tough. In this week's parsha we just learned בוכה למשפחותיו and chazal say they were crying about the arayos that were now assur to them after Matan Torah...
Lusting over one's sister-in-laws is probably the most common lusting that a man struggles with his entire life. For a number of reasons. Firstly, having married her sister, the thought that we "could have" maybe married the other one is not unrealistic in our mind (even though we all know it's ridiculous). Secondly, we tend to compare her to our wife and think, "Hey, they come from the same parents, so why isn't my wife so pretty or slim?" or whatever... And lastly, because we keep seeing her. She's always around, at Shabbos meals, Yom Tov meals, Simchas, etc...
So yes, you're not alone. This is a big struggle for most guys. And therefore, you must utilize all the tools in your toolbox to be misgaber on this, such as Teffila, making Gedarim, constantly being mischazek, etc... And when you're at the table or at a simchah, do you best to sit in a way that you'll see her the least possible.
P.S. Two things to keep in mind: