Deb, that's terribly sad.
Only thing I can think of would be some creative brainstorming about how you might see those children without being subjected to ongoing contact with their mother.
I wonder if there's any way you could offer her the gift of your babysitting, or better yet, even offer to take JUST HER KIDS with you on a vacation? You could maybe even literally make something up about some special deal where you are able to bring extra kids, but not adults. Or make up a special trip that is just for you and your H, but you've decided to bring kids along for your kids to play with? You could make it a purposeful time-for-just-H-and-me kind of thing...
I think what matters isn't that they have a name for what's wrong with their mother, but that they get a chance simply to experience loving, nonmanipulative behavior from a mother-aged adult woman.
If you do get that chance, I'd just love and appreciate and be affectionate and laughing with them, just give them that experience if there's a chance to. As to insight about their mother, sometimes one adult can say something very truthful that would ease their hearts without stabbing their shaky safety (which is still dependent on being with their mother, darn it). Like:
Bringing up a topic about unstable or weird adult behavior, and then saying very gently, you know, when I was little, I didn't know that adults could be all mixed up. I thought they were always right! (amazement...)
Anyway, I know it may not happen, it may just be too much. And I know you have to protect your core self from any relationship with an N. It's hard to watch kids, esp. N's kids...
Bless you for caring about them, Deb...
Hops