SD has taken up bullying YDS who is 7 years younger than her.  I did notice she scapegoated him when she decided he was the reason she didn't want to be here anymore.  Since, I totally missed that entire interraction, because I try to avoid SD, I decided I would make sure both my kids are supervised around her (DH is codependent so I can't trust him).  She's always been a model older sister despite the way she has treated me, I never dreamed this would be an issue.

I ended up having to explain why my kiddos had to come when I took them out with me, and we had an enlightening discussion, and came across the issue that she is consistently is trying to get ODS to gang up with her to bully YDS, and YDS feels like every time he tries to get back at her, he gets in trouble with DH, and SD gloats.  (This was her exact interaction with me, goading me into arguments with her, then gloating while DH got after me.)  That very evening at the dinner table, I saw it myself and shut it down via my kids (because me disciplining SD in any way turns into WW3).

I had it out with DH, and of course he promises he will pay more attention, but reality is, he has no control over SD, he still hasn't even effectively dealt with the way SD treated me, so I'm not sure what he plans on doing exactly, since she's almost grown.  He did focus on spending more time with YDS this weekend, and SD subsequently holed up in her room.  But, I think that's SD's issue with YDS, YDS and DH get along a little too well for her liking.  Really I think it's just that YDS shares hobbies with DH that the older two don't, and he makes the least amount of demands on DH, where as the older two have more finicky personalities.  

So, I'm at the point where I just want to take my kids and leave when she's here, because I already do not like being in the same room as her.  I absolutely hate having to supervise her around my kids.  (I totally have the power to kick her out, but I just don't think this quite justifies it... she is mentally ill).  I'm thinking next time she starts something, and DH does nothing, I'll give her a piece of my mind, pack my kids up, leave town for the weekend, and let DH to deal with the blow up without me.  Maybe that will motivate some change, maybe she will simply just not come anymore and choose to live with the dysfunctional side of the family where that kind of behavior is acceptable.