How to deal with a BPD spouse.

Hi and thank you for reaching out!   First off I want to acknowledge how challenging it is to navigate mental health issues with family members, especially if they are not at the point where they are willing to get the help themselves.  And it only does make sense that with your mother-in-law moving in, that you are now beginning to feel the effects of it all.  

Even though in an ideal world, they would both be in therapy and learning ways to manage their BPD, however with that not being the case, the focal point should be on what you can do so you don't feel that level of guilt or that depression you described feeling. Would you be open to therapy?  In the very least, therapy for you could provide you with the tools to help you be able to cope with your wife and mother in law's behaviors on a day to day basis.  Especially in seeing your statement about not knowing what to do, this could be a better good place to start. 

Another thing that I would also recommend would be some psychoeducation on Borderline Personality Disorder.  Psychoeducation basically means any type of information that will help us better understand something, in this case BPD.  The hope with providing psychoeducation is that by helping us better understand what something is, we will be better able to cope with it.  Not sure if this is something that you have already spent time learning about, but I think having resources can be very insightful.  Here are a couple of resources for you to take a look at:

https://www.yourhealthinmind.org/mental-illnesses-disorders/bpd

https://aamft.org/Consumer_Updates/bpd.aspx

https://www.Dbtselfhelp.com

The first two resources provide a really good snapshot of borderline personality disorder and the third gives a little insight into one of the treatment modalities that is utilized with people who are diagnosed with BPD.  What I really like about this modality of DBT (dialectical behavioral therapy) is that anyone regardless of diagnosis can benefit from using these skills. 

In the second link provided, they list several resources at the bottom of the website, one of them being BPD family, which I think you could really benefit from. This could be a wonderful opportunity to connect with others who are dealing with similar concerns and hardships as well as help create a network for you so you don't feel alone in all of this.

I know all too well how challenging it is to navigate relationships with people who have untreated mental health issues and at the end of the day, just remember to surround yourself with the support that you need.  I hope this has been helpful.