The trauma was real and involved the sudden death of a family member. Her behavior has been ever more confusing, egregious and hurtful because it is directed at the spouse and other family who were most affected by the loss, and who clearly need to prioritize the deceased's children and their own mental health. No regard for this, rather more rage that she is being "ostracized," when in fact her behavior has extended and amplified the pain, broken up the family, and stolen attention from the children who need it (who of course, she claims to love and is also enraged at being cut off from them due to her behavior).
I am sorry for your loss of this family member.
My BPD mother's behaviors were extreme- but in context of my family of origin, and minimized, normalized. Also, she was enabled and my father did a lot of emotional caretaking and we kids did later as adults. She also was able to "hold it together" in social context.
My parents were in their elder years when father got ill and eventually passed away. BPD mother's behavior escalated and like your sister's it was the opposite of what we would have expected, what we think as "normal". While what we experienced in our FOO growing up may not have been "normal" behavior, we didn't know another example. As adults, we have seen the "normal" as people responding to such a situation by being empathetic and supportive.
Why is this apparent now?
As CC43 posted, the pwBPD is so overwhelmed by their own feelings, they don't see the situation from other people's perspective.
Karpman triange dynamics helped me to explain this better. This doesn't make the behavior acceptable, but it helped to understand why it is happening.
https://www.bpdfamily.com/content/karpman-drama-triangle.
My mother (and I think most people with BPD) see relationships from victim perspective. Other people are either rescuers or persecutors. A person can take on any role in the triangle.
The usual family dynamics had BPD mother in Victim position. Dad mostly in rescuer. Us kids as his co-enablers. When Dad got ill,the focus shifted to him, he was the one who needed help and caretaking.
BPD mother, also stressed by the situation (understandable) perceived this response as "taking away" the usual caretaking and attention from her. Her own emotional distress increased with the situation and less caretaking. PwBPD project their own emotional discomfort as coming from someone or something else. It isn't factual but in the moment, to a pwBPD feelings appear as facts. Dad - and also us kids- to her, were in persecutor position now, hurting her on purpose.
So, she lashed out. Instead of being supportive and empathetic, she lashed out. It was destructive.
For other people, having seen "normal" and expecting "normal", this behavior is shocking, confusing. It would be understandable that people pull away from the pwBPD for their own emotional protection. The pwBPD may see this as abandoning them. For you, it's seeing your family dynamics from an adult perspective.
This doesn't excuse the behavior. At these times, other people are quite emotionally distraught, and may need to have boundaries with it. You may need to do this too. However, I think if you can see this as a behavioral pattern, rather than personal, it may help you and your feelings.
Sadly, this is a hard situation for affected family members to reconcile, and it's too soon to know if that is even possible. At the moment, the affected family has more going on than anyone can manage. They are the ones dealing with a huge loss. Your sister isn't capable of being empathetic to them. This is going to go the way it possibly can and that may take some time.