Hello and a warm

Glad you found us. Family learning and family growth can certainly help improve the situation for a person with BPD (pwBPD) -- or, at minimum, can help the family stop accidentally making things worse.
It sounds like your D20 has a diagnosis and is in treatment? Does she agree with the diagnosis, and is she compliant with treatment/going regularly?
From what I've heard on the boards here, parents and family members have had varying degrees of success with being the person directly implementing DBT skills with the pwBPD. Sometimes the relationship is too close and it's not successful -- some pwBPD need a little more distance in the relationship (like with a therapist/someone more neutral) to be able to take in the skills. Other parents do have some success with reminding their adult child to do things like self-care, deep breathing, and other skills. So, it depends.
It may still help, even if you can't be the one
teaching your child DBT, for you to educate yourself about the approach and skills involved. There should be plenty of free and reputable resources online for family DBT education; a great place to start (besides
our section of articles!) is the
National Education Alliance for Borderline Personality Disorder. Their
Family Connections program is well regarded.
Is your D20 living at home? Do you have other children, too?
Keep us in the loop -- we'll be here for you;
kells76