Is AUTHOR wrong in the post ? POST : Here's the deal: my friend's 30th birthday is going to be within a few days and his wife is throwing him a party at their place. She invited me and all of his friends to a facebook group in order to invite us. I agreed enthusiastically to her proposal because I really wanted to be part of his 30th birthday.

It was only after having a closer look that I realized that my friend's sister is also part of this facebook group. And only a few minutes after I realized this she wrote back that she and her fiancé would attend as well. Now here's the thing: some two years ago me and her had an affair that lasted for almost a year. Things were kind of intense and by the end of said year she wanted to take it to the next level, i.e. have an actual relationship with me. 

While enjoyed being with her, I just didn't have any feelings towards her. I liked the sex, her company and everything about her - but I just didn' have any feelings towards her...I couldn't explain it. It was then that she told me that she wanted to end the affair because of my lack of commitment. I understood her position and we agreed to part ways. There was no hostility or fighting. We agreed to remain in touch, though. 

Now, I know it sounds stupid: but a few monhs afer we broke off, I started to miss her terribly and regretted it immensily that I could not commit. But by that time she already hooked up wih anoher guy and they're now engaged. 

It seems that I had feelings for her nevertheless - and might still have! I still fantasize about her quite often. And it is therefore that the thought of having to spend an entire evening in the same room with her and her fiancé makes me feel terribly uncomfortable. If I had realized that she had been invited as well, I would have come up wih an excuse and declined the invitation. In this case, however, I agreed to attend without checking beforehand whether she would attend as well. I just don't know how I will react in this situation: I might be horny, angry, drunk or any of the above. 

Because of that I'm considering of coming up with an excuse (i.e. a lie) and tell his wife hat I cannot attend. WIBTA?

Tl;dr: Agreed to attend a friend's birthday party without knowing that his sister and her fiancé would attend. I once had an affair with said sister and still have feelings for her: I think about cancelling my plans to attend his party, which makes me feel kind of shitty. 

Also: nobody, not my friend nor his wife, nor anybody in our mutual circle of friends knows that I had an affair with my friend's sister. 

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