Hello Anthony: Thank you for bringing your concern here to PC. I see this is your first post. So... welcome to Psych Central.

I'm sorry you are struggling with this. And I don't know as there is a lot I can offer you in the way of advice. Clearly the best thing to do here, I would think, would be to try to forget your love for your wife's sister & be content with your wife & your marriage. Tell your wife's sister you were wrong to express your love for her & that neither you nor she should speak of it again. But of course what might be the best thing for you to do, & what you can bring yourself to do, may be two different things.

Personally, I think what you may need to do here is to find a counselor or therapist of some sort with whom you can talk all of this through at-length & in-depth over a period of time. Opening up to someone, preferably a mental health professional I would think, may be the best way to work through how you're feeling & decide what to do about it. (To my mind the situation is sufficiently complicated you're unlikely to find the solution on-line.) The worst option, to my way of thinking, would be to simply allow how you're feeling, & the thoughts you're having, to just continue to rumble around over-&-over in your mind. That is, it seems to me, a prescription for ongoing despair & anxiety.

In the meantime... here are links to 4 articles, from Psych Central's archives, on the subject of rumination. Hopefully some of the suggestions in these articles can be of at least some help in dealing with the thoughts that are tumbling around in your mind as you try to figure out how to proceed:

When Ruminating Becomes a Problem

The Reasons We Ruminate and How to Reduce the Cycle

8 Tips to Help Stop Ruminating

Why Ruminating is Unhealthy and How to Stop

I hope you find PC to be of benefit.

P.S. Here is a link to another article, from PC's archives, that (hopefully) may also be of some help:

Putting the Brakes on the Impulse Toward Infidelity | Surviving Infidelity