Hi Mollhia:Welcome! I'm so sorry about the situation with your SIL. BPD behaviors can be very difficult to understand and manage. It has to be very frustrating for you and your husband. The meltdowns can be very uncomfortable and can take us by surprise if we haven't witnessed them before.
I also attempted to reach out and apologize for yelling, but she never responded. Does anyone have any thoughts on how to repair this relationship?
A person doesn't have to have a BPD diagnosis (or qualify with the number of traits), a few strong BPD traits can make someone a very difficult person. Vacations seem to set the stage for a dysregulation event.
It's sad that she doesn't want help and can't recognize that she has a problem. Unfortunately, you can't help someone who doesn't want help. What you have control over is yourself, your boundaries and how you react and interact. Best for you and your husband to prepare for a future meltdown. It can be hard to not react, but it is best to not engage and remove yourself from the situation.
Not everyone with BPD traits is the same. It's common for them to NOT want to have a warm fuzzy conversation about their behavior. The length of "the silent treatment" can vary. At some point, they may just carry on, as if nothing ever happened.
If she lives close by, perhaps you might want to try and contact her again, with an invitation for lunch or dinner out somewhere. You can decide if it's best to have just the two of you, or to include your husband. If she still ignores you, after an invite for a meal out, you could send a follow-up note and just say something like: "My offer for lunch/dinner is an open invitation, just get in touch with me/us when you are ready. Hope all is well with you"
It's hard to say what she will do. Some people with BPD traits might get over an event quickly, while others are grudge holders and can paint people black (you are on their poo poo list) for years.
You may have to wait until there is some family event that she attends in the near future. Has she been speaking to her parents? Any negative things happening in her life currently (job? boyfriend?)