Difficult Sister in law.

Wow she does sound difficult! Feels to me like you're giving her too much of your head space and mental energy. Boundaries are important in families, and I think you need to construct some when it comes to her. Try a gradual fade vs a declaration of anything. With these types, drama can start over nothing. This is less about your wedding and more about your future in this family.

If your fiancé wants her in the wedding party, she can stand on his side. Tell her it's trendy to have mixed wedding parties, lol.

I'm really sorry she's like this. I think polite and distant is really the way to go here. I hope it works out.

"The slamming my foot" comment is just to express my intrusive want to say that, but I won't.

Thank you so much for your time, I appreciate your advice!

Sadly, she does have a lot of my headspace. I care about having a good relationship with family since I know how limited in family I have.

I do need to have boundaries, I thought after 11 yrs of being in her life, I could let her in more and be comfortable, but I'm being taken advantage of.

We don't have a wedding party, she didn't either. I was going to extend an invite for her to be in my bridal suite while I get ready, and I decided no. Why? because I wasn't given the same.

Polite and distant it is.

That's sadly my intention, as much as I want us to be super close. :C

As someone who's been dealing with a difficult SIL issue for over a decade now - the only answer is polite but distant. That's how I deal with mine. I'm polite to her when I see her, but she's not involved with my wedding at all, and outside of when I have to interact with her in person, we don't have a relationship. I make sure that I am there for her kids, I send them presents for their birthdays and holidays, but she is not the person I'm concerned about having a relationship with.

Don't let her ruin your day with her attitude. I think your instinct to not have her get ready with you is smart (She'll most likely try and make it about her or spend the whole time comparing what is going on her what she did at her wedding and you dont need that negativity).

I totally get wanting to have the great relationship - I always did too. But I learned a long time ago, my mental health is worth more than a relationship with my SIL. And don't forget - family doesn't always mean blood. Your chosen family can be worth more than their weight in gold when your family is not great, or gone.

I'd definitely limit how much you tell her about anything wedding-related. If she asks just say you don't know, it's a surprise, or that you're still working out a few details. The less she knows about, the less she can have an opinion about. I totally get wanting to have a great relationship with your in-laws, but sometimes you just have to keep people at arm's length for your own sake.

Your SIL sounds a bit narcissistic. Agree with the comments that have advised to be polite but distant. Don't give her info about your wedding. If she asks, deflect. "We're still figuring it out." If she offers to help, politely decline. "Thank you, we've got it handled!" ...etc. I would also be gradually less available to respond to her emotional needs, as it doesn't sound like it's been good for your mental health to allow her to use you for those purposes. But whatever you do, don't make a dramatic exit or a big confrontation out of it. If she is a narcissist, she's looking for a reaction from you, so blowing up at her will only keep her coming back for more (and trying to drum up support for herself within the family and turn them against you - beware if you become less available to her she'll probably do this anyway, but it will be easier to rise above that if you haven't given her any juicy stories to tell).

FWIW, when I got married, my brother and his girlfriend had been together for probably 8-9 years and had a baby together. They still have no plans that I know of to get married or engaged. It never would have occurred to me to exclude her from anything because she is my brother's partner, therefore she is part of the family. She was a BM in my wedding, was in all the family pics, etc. Also, my husband's father died before I "officially" joined their family - we had been dating somewhat on and off for ~2 years so I'd only known the family maybe a year and a half. I went to the funeral with the family and was seated in the front row with them. I say all that to say, unless you have done something that makes your SIL feel uncomfortable having you around (and it certainly doesn't sound like it given she's crying on your shoulder about her marriage problems), she has no reason to exclude you just because the two of you aren't married. It doesn't seem like it's really about that anyway - it sounds more like she's threatened by you and afraid you will steal some of her family's attention away from her.

Yes, I know you would have wanted something sincere and genuine but this is not what that is. Doing some work around acceptance of "what is" might be helpful for you.

Yes, she doesn't belong in the suite while you get ready. You want people with true happiness for you around you.