Hope,
I can relate to blow-ups at family gatherings! It's a shame, as it seems impossible to experience a family gathering with my diagnosed stepdaughter, without some sort of blow-up.
What I've observed includes the following:
*Untreated BPD involves an extremely short fuse. Almost anything can set it off.
*Difficulties in other aspects of a person's life--with friends, schooling, homelife, love life, work, etc.--can "prime" a person with BPD. When "primed," an innocuous comment or body language can set them off. The bad feelings, disappointments or frustrations materialize, seemingly out of nowhere. They were just simmering underneath the whole time. Maybe they emerge as projections!
*Untreated BPD comes with low tolerance--of others, of frustration, of boredom, of stress, etc.
*BPD involves a trauma-like response to ordinary stresses. By trauma-like response, I mean a fight-or-flight response to an annoyance. So if somebody asks her to please straighten up her bathroom because guests are coming, or to refrain from snacking on an ingredient reserved for a special recipe, she'll have a tantrum or isolate in protest, because she's accustomed to doing what she wants whenever she wants, and she resents being asked to do something she doesn't want to do, especially if the "guests" seem to be more important than she is.
Family gatherings are so tough, because they are outside of the routine, and they involve people who are important from an emotional point of view. Thus the emotional stakes are high with family gatherings. My diagnosed stepdaughter will struggle with any of the following:
*Not being able to entertain herself with screens as much as she wants during family events.
*Seeing others be joyful. She feels bad, and she can't bear to be happy for others.
*Mixing with others with whom she's had blow-outs in the past. She blames them, and they are the enemy; she deems many family members as traumatizers or "abusers."
*Feeling that others are judging her. Questions like, "How is college going?" or "How's your summer job?" are perilous, either because she's quit and feels shame, or she feels that others are judging her negatively.
*Comparing herself to cousins/siblings. She sees herself in a negative light, even if nobody else does. If her cousins/siblings talk about achievements (a new job, a new romantic interest, graduation), she feels inferior by comparison and is prone to blow up or leave the scene.
*Tolerating small talk with older folks. They are just too boring.
*Partaking in conversations. She will only give one-word responses to direct questions, and can't seem to engage in or hold a conversation.
*Not being the center of attention.
So yeah, I wouldn't go on a vacation with my diagnosed stepdaughter. She can't handle a morning or afternoon with family yet, she's just not ready. A whole vacation would be a pressure cooker.
Can you identify the triggers? I think I've identified major ones in my lists above. But can you avoid them? I don't think it's possible. Avoiding stress is impossible in the real world. Maybe it could be avoided for an hour or two, if everyone is on their very best behavior and treats the person with BPD with kid gloves, but that's not a realistic scenario for most families. At some point the person with BPD needs to develop some emotional intelligence and resilience. That seems challenging if untreated and still in the blaming-and-hating-everyone-else phase.