Hello alxpitcher,
I am so sorry you are having to deal with this alone with your husband gone at the moment... .
How was the relationship with your MIL in the past when your husband was around? Was she treating the children in the same way or is it something new?
Are you worried for your children's physical safety, or their emotional well-being? In either case, you have the right to protect your children. It is a difficult situation as you say that you are by yourself at the moment and also that you feel an obligation to keep in touch. What does your husband think about it in general?
Understanding BPD and learning to deal with the person suffering from it takes time. There are ways that can improve our chances at better and calmer communication, and also there are ways to protect ourselves from the harmful behaviors. You will find tools and skills in the resources on this website that will help you navigating these situations.
Should I be concerned? What steps should I take? Or am I just out of line?
In general, yes, you should be concerned. However, it is an issue for the long-haul, and with the right resources, you will be able to protect your children and help them navigate the complicated relationship. People w/BPD can be very loving at times, but they aren't always safe to be around. In the short-run, if you feel your children are not safe, I would remove them from the situation every time as calmly as possible (you can even make up excuses before you get confident with one of the most important of tools:
Boundaries.) As time goes by and you learn more and become more confident, you will be able to start setting boundaries on your MILs behaviors (doing that right away is not advisable, because setting boundaries causes a push-back from the person w/BPD and that can be rather unpleasant and unproductive if we aren't ready to deal with it).
In the meantime, learning as much as possible about the disorder and the communication tools will send you in the right direction. These might be good ones to start with in addition to the Boundaries above:
Validation,
S.E.T. - Support, Empathy and Truth,
Examples of boundariesAlso, setting up a support network of people around you (family, friends, church, therapist, this website etc.) and taking good care of your own physical, mental and emotional needs is essential.
Welcome again, alxpitcher, keep posting, the members here are willing to help and many of them have been in your shoes so they understand how you feel.