Hi DLE123,
Welcome and hello

I'm glad you found the site, and sorry for what brings you here. And glad your D16 is safe and getting treatment.
BPD in Adolescence by Blaise Aguirre is a very helpful book. He is a leading expert on BPD in teens and also has some useful youtube videos out there.
I found Buddha and the Borderline to also to be helpful. It is the story of a woman who went through DBT and was in recovery after she made it through the program. It can help you understand DBT from someone who went through the process. I also felt a lot of compassion for people with BPD in reading her book.
Our own relationship and communication skills can help, too. They won't always make things better, they can prevent them from becoming worse. Loving Someone with BPD by Shari Manning is helpful, and so is Valerie Porr's book (name escapes me).
Is your D15 staying in a residential treatment center? Do you have a good support network while you both experience this difficult transition? It can be essential to have a therapist of your own -- this made a huge difference for me as I learned how to communicate with my son, and most important, to learn how to take care of myself so we weren't both falling apart.
Keep posting. It really does help
You're not alone.
LnL