Hi Grammyto5
I like your screenname

Welcome to bpdfamily.
Not knowing what you are dealing with can sometimes make it very hard to determine how best to move forward. Now that you know about BPD and are educating yourself about the disorder, this will hopefully enable you to cope with this all better. Great that you had that friend to talk to and now also have a therapist to guide you through this.
My question is:  :)o I confide in family members that our DIL probably has a personality disorder? It has helped me to think of it as she's ill.
I am not certain if it is a good idea to share this with your extended family-members. Perhaps it's best to let them come to their own conclusions and stay out of whatever relationship they have or don't have with your DIL. I think it's clear to everyone involved that your DIL has certain issues no matter what label you put on it. My advice for the immediate future would be to focus on yourself and your own well-being and the relationship you have with your son and DIL.
When dealing with uBPD individuals you can often quite easily lose yourself and forget about your own well-being as you try to make sense of the confusing BPD behaviors. That's why I think your friend's advice of self-care is very good