Recommended reading for a kid to understand?

[worriedStepmom]

We are fairly certain my stepdaughter's mother has BPD.  (She's only been diagnosed with an anxiety disorder, but she meets all the criteria.)

SD is 11.  When we realized how bad things had gotten for her a few months ago, we stepped in.  My husband now has primary custody (65/35) and SD is seeing a therapist.

With the therapist's help, SD is doing a great job of setting some boundaries with her mom, but the emotional manipulation is very very strong.

SD knows about mom's diagnosed disorder and that mom has a therapist.  We're struggling with whether to tell her that we think her mom meets the criteria for BPD.  That would provide some more context to her about some of her mom's behaviors and potentially help her understand that it's not her fault, that her mom isn't necessarily choosing to be cruel, and that mom isn't necessarily going to change. 

Do y'all know of any age-appropriate books to help explain to her what that means?  Everything I've seen seems geared to adult children.  Or, at what age would you be candid with a child about what you think is going on?

[takingandsending]

The risk you run is that if mom is undiagnosed, then SD may unwittingly mention it and precipitate a lot of backlash on SD and you. It's better to talk about behaviors, validate SD that way and also arm her for dealing with an emotionally manipulative mom. That's what I am trying to do with my S7 and S12.