Welcome to bpdfamily Nightbaby
BPD is quite a challenging disorder, but now that your son has been officially diagnosed you at least know what you are dealing with. What led up to your son getting this diagnosis and will he be getting targeted treatment for his BPD?
You mention your son's verbal and physical behaviors. Is he physically violent towards you?
He reminds me of my father, AND my mother, both long passed away now, and my living sister as well. I learned to cope with them all by proactively doing everything possible to keep them "happy" most of the time and dodging/running/hiding/wondering what's next, the rest of the time.
What you describe here is what many I think would call 'walking on egg-shells'. There are however other things you can learn, other and likely more constructive coping mechanisms. I too think it is good you found this site and definitely encourage you to take a look around, read others' posts and explore the various resources.
You indeed might be powerless to change your son and sister if they do not want to or are unable to change, but what you can change is your own behavior and how you respond to them. By changing your 50% of the relationship, you will influence the entire relationship, regardless of whether the other person changes or not. Do you feel you've been able to accept the reality of your son's (and sister's) BPD and the consequences this has for the relationship you are able to have with them?
The Board Parrot