Hi 3Hearts: Sorry about the problems with your daughter. Were you aware that you were pregnant with twins from the start of your pregnancy? At what point was the twin lost?
You may already be aware of the website below. Someone from the UK conducted a study regarding Vanishing Twin Syndrome and traits that resemble BPD.
www.wombtwin.com/wtsarticles-borderline/4580514969What other BPD-like traits does your daughter have, in addition to emotional dysregulation? Has she ever been treated for any mental health issue before? (therapy or meds?) Does she live on her own and hold a job?
Is your daughter receptive to going to therapy? Not everyone with BPD behavior is the same, or has the exact same group of traits/issues. I'm thinking it may be beneficial to start with a therapist that has experience with BPD, until you find one you think might be more suitable. If you daughter doesn't have any memories or specific feelings about a twin at this time, then it might be helpful for her to gain some tools to deal with managing her emotions and learn healthy ways to self-soothe.
If issues come out during therapy about a twin, then that could be explored in some way.