I have been going through a bunch of family shit the past couple years, and I just got engaged a week ago. I have quickly been knocked down off Cloud Nine by members of my family immediately asking if I will be letting my father's piece of shit mistress/girlfriend attend my wedding. Long backstory:

My father walked out on my mother (after 4 kids and 30 years) to be with a mistress. He really broke all of our hearts by lying. This was only a couple of years ago, but my mother has already re-married to a guy I really like. My father has been emotionally manipulative and abusive to my siblings and I. He yells at us if we are not "nice enough" to her, but his definition of "not nice enough" is difficult to predict. He loses his temper when he decides we've been rude, when we are trying as hard as we can to bite our tongues and keep the peace. We are so accustomed to him flying into blind rages that we know not to rock that boat, and that there is no reasoning with him.

We know that his girlfriend is the mistress he left my mom for. Now, I realize that 1) my parents are better off not together and 2) I can't stop my dad from pursuing who he loves. It is the lies and the continual lying that hurt. She is an intruder to my family. I know the home-wrecker is not at full fault for the circumstance, but when I look at her I see someone who was able to convince my dad to completely destroy his family and abandon his children (he fell out of communication for six fucking months). I can try to handle him but she needs to stay the fuck out of my life right now until I can sort through shit with him. He has as much as told me that I am not allowed to see him without her, that she is family now (she's not, he won't fucking marry her - he's too afraid she'll get control of his finances like my "whore mother" and all her child-raising did). He told me that he and I will not have a relationship unless it goes through her, that I am to treat her "as I would my own mother." He shoves her down our throats when we are not ready for her.

However, since they both act like nothing has happened, it's a little bit difficult to get that point across. You guys are probably familiar with the strategy of liars: if they don't admit to something, no matter what, it is not true, and you are the shithead for holding them responsible for it.

Not only is it obvious she is the mistress from circumstances, but my father admitted as much to us, and then took it back and now acts as if we are vilifying this woman for no reason. He and she are the victims, and we are the brats - he is desperately trying to rewrite this narrative and we can see that. My grandparents (father's parents) have been very unhelpful because they want so badly to keep the peace. While they understand and accept our reluctance to let this woman into our lives, there is nothing they will or can do to talk some sense into my father. That ship has sailed. He is unreachable - and it has taken a lot of therapy for me to admit that to myself, and while they don't say as much, I think it has been a similar struggle for them, to see their son so lost and so actively destroying his relationships with his children through lies and rages (he hits below the belt when he is angry, he says things that cannot be forgotten).

So here is the real problem, now, based on the fact that my father cannot be reasoned with: I recently got engaged and I am ecstatic about that, but I am dreading the wedding already because I know my father will refuse to attend without this woman. However, I am one shade away from an emotional breakdown whenever I have to see her, even when I picture her in my mind. I absolutely cannot have that feeling at my wedding. Not only that, but it is tremendously unfair to my mother and siblings to subject them to that, as well, because we all have a hard time with it. I am not going to have a good time if I know my mother is about to break down, either. It is a no-brainer, to me, to invite my stepdad. He has been such a great addition to our family, but that sets a precedent that the mistress should get an invite too.

I really want to tell my father she is not invited, but I am terrified of the consequences of that. Alternatively, if she is at my wedding I am really afraid it will just feel awful to me, that I might break down and cry, and that my mother will not be able to handle it. I will also feel awful if my father is not there, particularly because I am very close to his side of the family and I think it would be hard on them. There's also an outside chance he would just show up with her.

I feel like the Right answer is: make it clear she is not welcome, and if he wants to throw a fucking fit and sit out, he can. I just...I am not sure if that is something I will regret forever, or if it is unreasonable on my part.

I am 100% ok with him not walking me down the aisle, or all that shit. I have got two brothers and a grandfather and uncles that I love a lot more than my asshole father. Even if he came I may opt to have my brother do it.

I honestly feel like it will be impossible for me to be happy at my wedding if she is there. I can accept that I don't control whether she is at Christmas, reunions, graduations - but this wedding is mine and my fiancee's. I want to be happy. She is not my family. I feel like I should have control over who attends. What can I do? I feel so powerless, and it is making me not want to even get married. I don't want to elope. I need help figuring out where and how to draw a line. Am I required to suck it up and force myself and my wounded family to deal with this woman at my wedding, or is it on my father to sit out if he's going to have a fit? I can't decide which is worse. I wish my father would just agree not to bring her but I am positive that won't happen because of how invasive he has already acted with her.

I would really appreciate some encouragement or validation or advice or anything.