Im just wondering.....Would it be possible for your family to have a pre graduation breakfast or do something fun together before dd gets into town?
This is a great idea- or also do it after the graduation.
Karpman triangle dynamics helped me to understand the dynamics in my family of origin.
https://www.bpdfamily.com/content/karpman-drama-triangleBPD mother perceived herself in victim perspective. Others were either "on her side" (rescuer) or "not her side" (persecutor). Triangulation is when she'd talk to someone about a third person- and get that person to be "on her side" with the other one not on her side.
Sadly, this kind of thing did cause divisions in the family. If she rallied someone "to her side" "against" me- and I tried to clear the air with that person- they'd be stuck in the middle, having to choose.
Your younger D is caught in the middle and is trying to keep the peace. She doesn't want to have to choose between her sister and her parents.
These are difficult situations, they were quite hurtful at times. It doesn't seem fair that someone would do this with family members.
IMHO- and this is just how I see it- your best "defense" to this is to keep your strong bonds with your other children, without triangulating about their sister- so don't engage in discussions about her, which puts them in the middle. I learned that whatever I may have said to family members about my mother could potentially get back to her, as they'd feel a "dual loyalty".
Make your dinner with your D special time for the three of you. Make this a good celebration and memory. There will be less drama if it's just with your D. It's not what you planned but it still can be a good time.
You can also plan to do things with your other children, (your BPD daughter might decline or you may not wish to do this with her) each individually for their special days- birthdays, etc.
Don't change your family "all together" plans. This gives over power to your older D. I wouldn't exclude her by not inviting her. Leave this up to her to decline or accept. If she throws a tantrum- it will reflect on her. Eventually your kids, as adults, will decide their own relationship with her.