THIS IS THANK YOU FOR ADVICE!!!
Earlier in the week I posted the question what to do about my stepfather and his GF and so many of you gave such great advice. The post was removed because a few ladies were really disrespectful to me and to other ladies. (Which violates the terms of use here), so I couldn't comment there to thank you all. ***please I hope this doesn't start any more arguments. I'm posting in case other ladies are in a similar situation and maybe it could help them.
At the end of the day, I decided to just invite the girlfriend and suck it up. The advice I got was really good and helpful.
To recap--my stepfather and I don't have a great relationship. My mom is passed by 14 years and he has a new woman in his life, and she hates me. I am never invited to any holidays or anything so it didn't even occur to me that she would want to come. I told him that I was engaged and emailed him details about the wedding, he only replied passive-aggressively once ignoring it and even went so far to complain to my stepbrother, who I am super close to and is coming with his wife and kids, and my sister. It's been really stressful. We are getting married on the beach in a teeny-tiny ceremony and so I thought it was going to just be immediate family and again, it never occurred to me that she would even want to come as she literally hates me. Her daughters went to high school with me and apparently, they hate me too, even though I never even knew them. (Huge high school 3000 people)--thus the dilemma.
Our first planner said we could only have 17 people total, including us, the officiant and our guests so really we couldn't have her there, but we have since switched and we can have more people.
I didn't want to invite her. I don't want her to come. At this point, I don't want him to come. BUT I agree with about half the replies here that said that I need to invite her just to keep the peace, and it's proper etiquette. I talked it over with my FH and he is a bit worried that they will cause trouble, but is okay with it. I sent her a save-the-date with a note saying that she is invited since she wasn't included in the other one.
I'm grateful to all who replied, especially those who made really good points in views that differed from my own. With gratitude.