Hi again Travelling suse
Having done research online, my husband and I feel my middle Sil (my H has 3 elder sisters) displays significant signs of BPD. To the extent that I have ordered 'Stop walking on eggshells' (arrived today), found this forum, and am on the cusp of arranging to see a T with my H in order to help ourselves be free from the destructive behaviour that comes from my Sil.
Dealing with disordered family-members or in-laws can be quite challenging and really take its toll on you. That's why I'm glad you and your husband are able to support each other through all of this. When it comes to dealing with people with BPD, knowledge truly is power so I think it's very positive that you are educating yourself about this disorder

Having an extended support network in the form of a therapist is something many of our members have also found very valuable. What would you say are your SIL's most significant BPD traits?
You talk about her destructive behavior, was she always like that or was there a specific turning point?
Anyway, my Fil died 18 months ago and there has been a bit of a Hoo Haa about the Will.
I am sorry for your loss. What kind of relationship did you and your husband have with your Fil?
Considering all that has been going on with your husband's sisters and mother, do you feel like the two of you have been able to mourn your Fil's passing?
I have also responded to your other post and shared some resources there that I think you might find helpful.
Take care and I encourage you to keep posting and reading here