Hi Therapistaunt
This forum focuses on BPD behaviour so you’ve come to the right place. The first thing that jumps to mind from your post, is that the younger someone is the greater the success they have in therapy. Theres a section on this forum explaining therapies available. The biggest blockage is someone with BPD accepting any label that doesn’t make them sound fabulous. So here again your niece is ahead of the game if she shares the BPD suggestion. That all bodes well.
The reason they don’t diagnose at that age, and forgive me if you already know this, is that between 16 and 21 is where a lot of emotional development goes on, and hence its difficult to predict the adult, regardless of if BPD is present.
You mentioned its largely genetic. With NPD and most personality disorders, it also requires a certain type of upbringing, a stressful upbringing. BPD is a notable exception, where they believe around 20% didn’t have that upbringing, but that means 80% did. That said, BPD is often co-morbid with other aspects such as PTSD or other anxiety related illnesses. So reducing stress for someone with BPD is always a good thing. What aspects concern you the most currently ? Why has the last 6 months been so bad ? 16 is typically a stressful time for a child, could it be that ? Feel free to ask as many questions as you like. There's a great depth of experince on this forum, which I hope you can add too. Welcome to the forum.