Think of her as a vampire who needs to feed. That's what she's looking for- supply. If she's not getting it from you or the kids, she's not got much reason to engage.

Attention, reaction, emotions- that's the stuff she's looking for.

As tempting as it is, don't feed the beast. Even if she totally deserves it, you'll still come out looking bad. It wasn't until I fully understood the narcissist that I understood that old addage about wrestling with a pig. The pigs really do like getting dirty- they absolutely love it.