I think there are many posters here with similar type situations. I hope to offer some perspective, based on my own experiences. I am the adult daughter of a mother with BPD. Both my parents are deceased after a decades long marriage.
The Karpman triangle was very helpful to me in understanding the behavior.
https://www.bpdfamily.com/content/karpman-drama-triangleBPD mother was in victim perspective, and my father's main position was as her rescuer/enabler. For the two of them to be aligned like this- someone or something was in persecutor position. BPD mother's interactions with people were from her victim perspective- so "painting someone black" to another person who may be empathetic to her, and align with her was a common pattern. What is said isn't necessarily true. For someone with BPD, feelings feel like facts- so they may feel that way or- possibly be seeking empathy from the person they are venting to. This could explain why your DIL told stories about you to the midwife.
IMHO, and from experience, the most precarious thing about this situation is- that she is acting "normal" and may even be acting especially nice to your son. This is not malicious, it's fear of abandonment, as she may be aware that her behavior could push him away. PwBPD are very sensitive to shame- and so may not apologize or take accountability to avoid this feeling, which is very distressing to them- so instead they may act like "all is good now".
The person who has seen the behavior may second guess themselves and think well she's OK now. Also with my mother, there could be "reasonable" sounding reasons for the behavior- and your DIL may attribute her behavior to hormones and post partum. That may be a preciptating factor- but that doesn't mean her behavior was reasonable. It can give the impression that this was a one time event, but BPD is not a one time issue.
Why are you and your wife in a most precarious situation?- because, you are "on" to her and also because, next to her, you have the most influence on your son, her husband. Often the non BPD spouse's family members are likely to be placed in the persecutor position on the Karpman triangle.
You have no control over your DIL's feelings and your son will make his own decisions. Ultimately he will need to be the one who decides to carry out his plans and while you wish to be of support to him, keep the Karmpan triangle in mind and try to keep the drama in your interactions with her down, and keep your interactions with her positive- stay cordial, not emotionally reactive, even if you feel otherwise.