Should I invite my step-mom (who was the other woman) to my wedding?

If it's what you want. I'm in a similar position. My father was married to my mom from 78-99 and he cheated on her with one woman emotionally for years but then eventually moved forward and actually started having a physical affair with her. He then left my mom and married this woman the next year. They've been married for YEARS now and my mom passed away in 2010. I always said I wouldn't invite her because my entire family hates her... however, since my mom is gone I've really started to see how much our dislike has hurt my father and I can't lose another parent knowing that I kept a part of his life at a distance from me. So, we started to build a relationship with her and they will be here next week. It'll be awkward because my mom's sisters haven't met her but there BETTER not be any problems... it was 15 years ago!

@FMP I have forgiven her, and my approach towards her has been one of reconciliation. My issue is that she seems to want to act as though I don't exist, and she seems to dislike that I have a good relationship with my father. That to me is a problem. However I'll send an invite to both of them out of respect for my father, and it'll be up to her to accept or decline. Also someone further up I think said the affair wasn't my problem, but her coldness towards me is. I beg to differ that it wasn't my problem. As a child parents' issues DO affect you profoundly, and can even change the landscape of your life. I was fortunate in that I have a mother who refused to be a victim of circumstances, and by God's grace I'm better off after her and my father divorced. But my stepmother is a woman who in essence helped my father to break his marital vows, ignoring the fact that there was a child involved, and then now acts as though I'm the one who's intruded into their lives. I think that's pretty unfair...

I would probably be snide and put Dad plus 1 and not even put her name. I'm just a jerk and realize that this is totally a no-no in etiquette. Unfortunately, you might want to invite her, but definitely don't put her anywhere near your mother. I'd put them in the back or middle of the reception since there's no need to have them near your mom's side of the family. Although your mom seems like a pretty remarkable woman, there might be drama at some point seating your dad and his wife near your mom's family.

Edited: Spelling

As far as your last comment, your dad is the one who broke your parents' vows. Unless she was married to someone else at the time, she wasn't cheating. He was.

As far as her coldness towards you, it might be that she resents you on some level for all the years that your dad stayed married to your mom and came home to you even while having an affair and 3 kids with this woman. Is it your fault? No, of course not, but it is what it is.

I agree with kahlcara -- her coldness may either be (1) just her personality or (2) leftover resentment from being the "other woman" for so long.

My dad cheated on my mother multiple times and ended up marrying one of the women. This was all 20 years ago. My mom is still pissed about it but can be cordial when needed. I invited my dad and his wife to the wedding but the wife ended up not being able to attend. Whatever. Regardless, I agree that you shouldn't sit your dad and step-mother at the same table as your mother. I had my father at his own table with his close family.