I have another theory. Maybe it fits with some of the others that have been offered. I have struggled with disciplining my children too, at various times and in various circumstances. The single thing that I have the hardest time with is getting them to clean their rooms. If I sit there and supervise them (without which they would just play and not get anything done, or make a bigger mess), I get too mad and risk being out of control. I get mad because they aren't working, or because I'm working harder than they are and they aren't doing anything, because I discover damage they have done that I hadn't noticed before, or because they have my stuff that I told the a million times not to take. I am afraid of my rage, which I never want to direct against my children. So it's best for me to keep my distance, even if it means I can't get the kids to clean their rooms.

I also have liked or even disliked my children at various times. There was a period during which I didn't like my son (the youngest), and didn't want to touch him or be touched by him. I really didn't know why, except that he reminded me too much of my brothers when they were that age, and I hated them at that age. I guess boys can be annoying when they are 3 or 4 years old. Eventually I got over it, and I found something I had written when he was a baby that reminded me that I enjoyed him as a baby more than I was able to enjoy my girls as bablies. I felt closer to him. I've had ups and downs with the girls too, but at the time it is very hard to see that my feelings have changed so drastically over time (weeks, months, years).

Rage is often an issue for people with BPD. So is the up and down pattern with relationships. Usually we think of that in terms of romantic relationships, but it is played out in any susceptible relationships, including with our children. Your DIL might favor one child this month or year, and favor the other one next month or next year. She may be completely unaware of the pattern.

Being able to understand the patterns is a step towards being able to fix the problems, but it isn't enough to fix it, and neither does it make it okay. Therapy may be more help if she would consider going. Therapy can also help family members to deal with the situations, whether or not the person who has BPD will seek treatment.