I love a good DBT group!
@Nighthawlk is right though - a "classic" DBT program, where you commit to it for six months (generally), do concurrent once-a-week therapy with a DBT therapist, and generally have 24/7 access to skills support - I did a program like that for about a month, and it was exactly like going to school. It was a ton of work, there was no sharing of feelings or support inside the group setting, and personally I found I didn't like my group or the leader. And then I was in an accident, so I stopped going.
I've also done DBT groups through Intensive outpatient programs - these are sometimes a M-W-F thing, or it can be an every day for a shorter period of time kind of structure. The ones I've done were essentially a phase inside of a partial hospitalization program. And all my experience inside those was really, really positive. The DBT stuff wasn't so rigid, they just stayed with the basic concepts and how they could be useful. It's frustrating, sure, when the group doesn't have a good dynamic or the leader is not strong.
But, as a way to be introduced to a whole lot of mental health concepts, and get some very practical skills, and also have a set place I needed to be every day instead of just never leaving the house....all this was really good for me.
So, a "classic" DBT group - very rigid, kind of cold, homework for days, but it really makes you do the work, and I do think DBT is a powerful way to attack certain kinds of problems. If your niece is diagnosed with Borderline personality disorder, she would benefit enormously from that kind of structure. I think there is no sharing specifically because DBT was developed for Borderlines, and the tendency to emotionally act out and/or ONLY express through emotion is something they need to curb. So feelings go in therapy, skills go in group.
A PHP or an IOP that has either CBT or DBT as part of its core - doing a few weeks in one of those can really be a great first step. it's support AND skills-based, you learn a lot of basic ways to talk and think about mental health, and everyone there has really different issues, but everyone is struggling, and they weren't dogmatic. That (for me at least) was a profound step in understanding that i wasn't the only person in the world with some of the feelings I had.