Hi concernedaunt
Welcome to the forum.
Although my sister insists she is not responsible for her behavior I still feel like there should be some consequences for her actions.
I encourage you to take a look at the top right hand side of this page. Reading and learning about BPD will better help you understand the daily challenges your niece faces and also her limitations. This knowledge will help you understand.
You've raised a tricky point that we all face. Family gatherings and, in particular, holidays and vacations can be very challenging for everybody concerned. My DS gets very anxious beforehand. If he feels something then it is fact. He can't help the way he behaves as he has a mental illness. It's not only embarrassing, it's a clear reminder to me that my adult child is different. My family don't understand.
Better boundaries and limits can really help family members protect themselves. It's admirable that you've come to the forum and you obviously want to try to find a way for you and your family. It really sounds like your sister and husband could use your support - I know I'd do anything to have my family at least try to understand some of my problems and the work I'm doing to learn new communication and validation skills to better interact.
Have you spoken to your sister and husband and explained how you're feeling?
Is your neice in therapy?
How does your sister and husband normally deal with the outbursts?
Sorry for the questions!
LP