I agree with Waves.

I also believe that she may need the connection with you.
I also wonder, and this is something you really need to consider.
Because she is over 18, the odds are that her dad and step mom
aren't in or allowed to talk to her psychiatrist. About what she
says to them.

Its more that I believe her dad needs to have a guide of when to come
home. I think your idea to sit with a counselor. WIth either the 3 adults,
to just learn how to help. And then all 4 of you to talk through things.

Is a fantastic idea. And honestly the reaction of her dad, is normal.
We always want to protect our kids. He needs to learn to let lose
just a little more.

Also I'm going to suggest you find your local NAMI alliance. I believe
just talking to others that have issues with family members will help
you personally. There is no reason you as a aunt, Right can't go
without the rest of the group.

Donna