Okay, there have been many theories regarding the mysterious deaths of both the Black Sister Ispan and the Brown Sister Adeleas. Many such theories center upon Vandene, Adeleas' twin sister. Not only was Vandene the first to see her sister after Adeleas' death and knew about her liking of tea; but she was at the area of the shielded Draghkar that attacked Moraine, making people think that Vandene shielded it. However these theories tend to be countered by the fact that she has a warder and wouldn't have been able to hide emotion when shielding shadowspawn. Also, since Adeleas was her sister it only makes sense that Vandene would be the first to find Adeleas. I propose a different theory.

Adeleas was very distraught at her sister's death and seemed to be maddened by the fact that Ispan had been killed as well. She was also surprised at the Draghkar being there, and shielded to boot. These are reactions that say she didn't know of the events, let alone committed them.

I say Vandene is the ultimate ace up the proverbial sleeve of the Dark One. When Vandene signed on as an Aes Sedai of the Green Ajah she had two personalities. One, who we shall call G-Vandene for sake of brevity, was a "normal" Aes Sedai of the Green Ajah. She was unaware that blacks were prevalent in the White Tower and disgusted by the very idea of Aes Sedai going over to the Dark One; however, her other personality, who we will call B-Vandene, was a believer in the Dark One's message. B-Vandene hungered for power and could see through G-Vandene's mind but only take over for short periods.

During these periods of control by B-Vandene, G-Vandene would remember only false memories of events that B-Vandene put there. B-Vandene joined with the Black Ajah and is unaffected by the Oaths because of that, but G-Vandene is still affected by the Oaths because it's her personality that took them. B-Vandene swore, perhaps, to Ishy himself, or some other high up Black. Then that Black was "silenced" and no one knew of B-Vandene other than Ishy or some other Forsaken. B-Vandene never comes out unless she sees something that could harm the Dark One, or herself that she can destroy, or something that can help the Dark One. Either way she would gain status in the Dark One's eyes.

In TPoD Ispan is being interrogated. Ispan is about to break and tell Adeleas of something that she shouldn't. B-Vandene takes over G-Vandene's body so that she can stop this, but before B-Vandene can to anything Ispan tells whatever the secret was. Now B-Vandene, acting like G-Vandene, takes Adeleas over to the shack under the ruse of talking about what they found out. While there she serves some sweet tea that Adeleas always liked. It's laced with poison and kills Adeleas. B-Vandene then goes over to Ispan and kills her. She uses her will to implant false memories of leaving the interrogating room into G-Vandene's head and leaves. G-Vandene then finds her twin and Ispan, both dead. This same thing, in a much simpler format, took place at Vandene and Adeleas's house. B-Vandene took over G-Vandene's body and shielded the Draghkar, then placed false memories into G-Vandene's mind.

The idea of split personalities could also help the Dark One after the great purge. If G-Vandene was ever made to swear on the Oath Rod again and was then asked if she was Black, she could honestly say no. Aes Sedai do not have to be correct when they speak the "truth," they simply must believe it themselves. G-Vandene would sincerely believe, know from her point of view, that she was not Black. She never swore or did any bad thing to her knowledge. B-Vandene would be safely tucked away inside her head, like Lews Therin is in Rand's head.

Also, G-Vandene's warders would not feel anything. Their bond is with G-Vandene and not B-Vandene so they would just feel the bond go fuzzy for a bit. This may confuse them at first but after a while they would get used to it and they wouldn't connect it with any bad events because A) all WoT warders are loyal to the point of stupidity, and B) not all Black operations are known, ever.

That's the theory. And it makes sense. Scary, no?