Yes, unfortunately it's a common pattern. It's good that your DIL still keeps a connection with your H. If you know this is a pattern- go along with it- for the benefit of your grandchildren and son.
I'm an adult child of a BPD mother. My parents are both deceased. There was a similar pattern in my family. My mother disliked my father's side of the family and in general- it was the female family members on both sides, (me included) who seemed to be the one "targeted" when she was angry.
If you consider projection, if a female is projecting aspects of herself on to someone else- a female person is a more similar model.
Another reason- they were "on" to her disorder- and this causes her to feel shame. The outward persona is a part of this. Fear of abandonment if someone really knows the parts of her she feels shame about.
Your H has an advantage if this is happening. He's male. He also seems to see her good side. It makes sense- I think we'd all prefer to be in contact with people who see our better side. In general, the males in my mother's circle seemed to be empathetic to my mother.
No much was known about BPD at the time but my father's family didn't say a word about her and did stay connected to my father and us kids through him.
My mother was controlling and my father also agreed to her wishes. Why he did this is complicated. It takes two to have a relationship like this. My mother also perceived people as being on her side or not her side- and making them choose her side. Read about the Karpman triangle dynamics. From my experience, the pwBPD is in Victim position. Your son is expected to be Rescuer. You are in persecutor position at the moment.
https://www.bpdfamily.com/content/karpman-drama-triangleThis is a pattern, it's less about you than the dysfunction. I understand you are grieving. This is difficult. Your DIL knows you are "on" to her. What she's afraid of is that you will influence your son to leave her, say something to her grandchildren or they will like you better than her. I know this doesn't make sense but - we can not change anyone's thinking and at the moment, your son is going along with this for the sake of the children.
But she left one door open- with your H. And the best chance of keeping this open is that you and your H go along with it. Let him see her with rose colored glasses. This is why he is able to keep contact. Contact is your main goal.
Do not react personally to what your DIL says. If you do encounter her by chance, stay friendly and nice. Sometimes the Karpman triangle dynamics change and someone else may be in the persecutor position. One doesn't know what can happen but emotions do change. We call being less emotionally reactive "grey rock"- as it helps lessen the drama. Just keep in mind that underneath her behavior is fear- somehow she feels threatened by you, even if it's not true, and to her, her feelings feel true.
