If your sister has BPD, she does not have the emotional stability to reason that your wedding is your special day and that your parents' attention is expected to be on you. What she senses is the change in their attention.
This could be anything, not just your wedding. It might have been if your parents were busy with something else, or someone had a cold, or for any reason not providing their usual attention to her. You are able to say to yourself "Mom and Dad are busy" for your sister, who sees things from victim perspective, she sees "Mom and Dad are purposely not paying attention to me".
This could be anything, not just your wedding. It might have been if your parents were busy with something else, or someone had a cold, or for any reason not providing their usual attention to her. You are able to say to yourself "Mom and Dad are busy" for your sister, who sees things from victim perspective, she sees "Mom and Dad are purposely not paying attention to me".
People with BPD traits (pwBPD) often never do get diagnosed. So we have to deal with what we see and experience and conclude accordingly. I remember my sibling was often sarcastic and seemed to have a chip on his shoulder, so to speak. I always called him the grumpy one but I have concluded he has never been diagnosed.
PwBPD are largely unpredictable but in some ways they have predictable patterns. They will typically have emotional overreactions to common triggers... weddings, funerals, anniversaries, family events, preparing for vacations, vacations, graduations, etc.
I agree with having some kind of contingency plan. One is to designate someone to pay attention to your sister. My BPD mother was older but one thing I did for a family event was to hire a one on one caregiver/companion for her. This person's role was to be completely attentive to her, bring her her meals, and be her companion. This person dressed up like a guest and no one was the wiser. BPD mother did know it was help- but it appeared like a friend at the event.
With your sister being younger, this wouldn't work but something similar- a friend, companion, relative- someone who can be focused on her during the wedding and not get into drama with her- and also someone who will take her out of the room if she acts up. PwBPD tend to hold it together in front of people, so unless your sister drinks too much or gets overly agitated, she may not- but the standby friend may help. It may be an extra guest at the wedding but the cost for that is well worth the peace of mind. Your sister may decide to not show up and also might change her mind at the last minute but there will be someone on standby however this goes.
With your sister being younger, this wouldn't work but something similar- a friend, companion, relative- someone who can be focused on her during the wedding and not get into drama with her- and also someone who will take her out of the room if she acts up. PwBPD tend to hold it together in front of people, so unless your sister drinks too much or gets overly agitated, she may not- but the standby friend may help. It may be an extra guest at the wedding but the cost for that is well worth the peace of mind. Your sister may decide to not show up and also might change her mind at the last minute but there will be someone on standby however this goes.
Over a decade ago one of our members faced a concern about her mother's likely behavior at her upcoming wedding. I admit, her approach was a bit more drastic than Notwendy's gentler approach. I dug this out of the way back archives.
I recall the past posts of a member here, Lexi, where she hired off-duty police as security to be ready to remove any people (her mother) who tried to take over or make scenes at her wedding. 

5 years ago I was a frequenter on this board as I was planning my wedding and watching my uBPD mom erupt like Mount Vesuvius... . Yes, they will rant and scream, but that will subside with time. I'm sure of it. The boundary gets set when they realize that you aren't a sure source of feedback to their bad behavior.
It was hard for me to really realize that what I was seeking at the holidays was something picture-perfect that was never going to exist. I spent a long time mourning the loss of things I never had--a mom who was able to treat me with love and respect. I had to learn to take things for what they are. You kind of need to go into things thinking of them just as acquaintances that have no right to hurt you...
It was hard for me to really realize that what I was seeking at the holidays was something picture-perfect that was never going to exist. I spent a long time mourning the loss of things I never had--a mom who was able to treat me with love and respect. I had to learn to take things for what they are. You kind of need to go into things thinking of them just as acquaintances that have no right to hurt you...



