Crush on Sister-in-law

I have a slight problem: I think I have a crush. on my sister-in-law. It happened a couple of weeks ago, I was dropping my kid off her place as she was supposed to stay there for a few days and I stuck around for a while, getting some coffee, helping her put her middle child to sleep, talking. And it just struck me. I was standing there, drinking coffee and talking to her while she was doing some dishes. And all I could think about was what it would be like to kiss her, how would her lips feel against mine and so on. And as we were talking, making eye contact, I got that feeling. You probably remember what it felt like when you were younger and got eye contact with that special someone? That feeling. Haven't been able to let go of those thoughts since, the ones about her lips against mine. It's really bugging me because I don't know what to do about it. It's not like the usual sexual fantasies I've had about her before, this is way different. That and I'm married and even if I wasn't technically married I still wouldn't want to risk my family. So I can't just tell her how I feel and be done with it as chances are she'll tell her sister (my wife) which might cause a situation. Not a huge risk, she is rather sensible and intelligent, but still. I just don't know.
Over the last few years, she (my sister-in-law) has always been very nice and friendly towards me and given me her support (if only verbally and out of my wifes earsight) when my wife's been verbally abusive (she gets that way especially when we're around her parents or when she's stressing herself up for whatever reason, usually unnecessarily. I tend to accept it to a point, depending on the surroundings). I guess that's why I like being around her, plus she's rather attractive, at least to me. So I don't want to have things become strange between us either as that would really make our stays at my wifes parents over certain holidays pure hell.

I think I already know some of the answers you guys will give me, I'm not pro-adultery (I'm not judging those who are adulterous though as I understand there are reasons behind every decision one makes), I just need to vent. I was contemplating writing an anonymous letter but haven't been able to make my mind up as to whether or not that's a good idea (I would certainly appreciate someone sending me an anonymous letter of that sort but some wouldn't. On the other hand, doing it that way I could vent a bit while also still being able to deny everything and just have a potentially uncomfortable situation instead of a total crashlanding if everything goes south)

PerKr