Finally.

I must have read 100 times over the past year that the way to deal with a dysregulated kid is to make short statements that name and validate what they're feeling. Yet I haven't been able to break the question habit. Yesterday, I finally did it, and lo and behold, it worked!

B was upset and being nasty to Z. I walked in and simply stated, "You're angry at Z." [Normally, I'd have asked him, "B, are you angry at Z?" and he'd have just gotten madder.] He sneered at me and said something snotty that was meant to convey "Duh, you moron!" Then I said, "You wish she'd go away." This time he actually said yes, with considerably less attitude. Then I said, "You wish it was just you, me and Daddy, like it used to be." He very calmly said yes, then "It was easier then." I agreed with him that it WAS easier then. Then I told him it was OK if he didn't like Z, even if he hated Z (which he says all the time). I told him those were totally normal feelings for a brother to have, and that I very often didn't like my own brother and sister when we were kids. It totally defused the whole situation! He stopped tormenting Z and sassing me, and our morning went on in a much more peaceful fashion.

I have learned a very big lesson.