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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:

1. She always looks “put together” when you see her around. But what you don't see is when she wakes up in the morning with messy hair and bad breath. (While our own wives we almost always see like that and realize they're human!)

2. In just a few years from now, your sister-in-law will be a mother of a few kids and probably be overweight and worn out, and you’ll look back and laugh at how you used to lust over her!