There has been a great deal of speculation regarding the question of Who killed Ispan and Adeleas? Lets start with Why were Ispan and Adeleas killed?
There seem to be three main possibilities. One, to prevent Ispan letting something important slip or to punish her for already doing so; Adeleas had either heard too much or was simply in the way. Exactly what this something important was is a matter for speculation, since we know little about what Ispan knew regarding the Shadow. However, if this was the reason, it came oddly late. Vandene and Adeleas had already been questioning her regularly well before the killing, and why the killer would have waited so long, increasing the risk that Ispan would talk, is puzzling to say the least.
Two, to increase distrust and discord within the group to prevent them from working together.
To me that seems like a more logical reason, not least because that is what has happened. If they were killed to keep a secret, then that secret must have been important enough to reveal that there was a Darkfriend in the group. But what if that was the whole point of the killing?
Quoth the Dark One, Let the Lord of Chaos rule.
The best way I know to create chaos among a group of people is to make sure that none of them trust each other. Well, the Aes Sedai dont trust the Kin, and the Kin dont trust the Windfinders and the Windfinders dont trust the Aes Sedai, and now even the Aes Sedai don't trust each other because they can't be sure who the Darkfriend is. So, if that was the Shadows plan, its working.
Is that particular group really important enough to bother with? Most of them arent, no, but three names spring to mind. Nynaeve... Elayne... Aviendha. Nynaeves caused some fairly serious damage to the Shadow, helping Rand kill Rahvin and keeping Moghedien a prisoner (and now, of course, her foray into healing saidin). Elayne and Aviendha are possible holds on Rand. But if they dont trust anyone then they cant get much done and they have to be constantly looking over their shoulders.
Plus, any group of channelers that large is a potential danger, if they can work together - as witness the Bowl of the Winds and the subsequent change in the weather. Its a good idea to make sure they cant.
The third possibility can be summed up as 'the Vandene theories.' Either one names Vandene as the killer, but for widely different reasons. The most common one is that Adeleas discovered Vandene was Black Ajah, or was about to, and Vandene killed her to keep the secret. The other is the reverse - Vandene discovered Adeleas was Black and killed her to keep the secret, and perhaps to spare her the shame and pain of being found out. Either way, she would then have taken out her anger on Ispan. Perhaps especially in the latter case, as she would see the Black Ajah as having taken her sister away from her, and see Ispan as its representative.
Kirstian and Zarya narrow down the possibilities to Merilille, Careane and Sareitha. This fits with what we know of the case; a Darkfriend Windfinder would most likely not have either means nor opportunity, and there are apparently no Darkfriends among the Kin, judging from how little Falion and Ispan knew about it. However, their logic does not rule out Vandene; even though she had both means and opportunity, almost any hunter would automatically exclude her from suspicion.
I find the second of the Vandene theories intriguing; it would certainly make for plot and character development in the upcoming books. However, I'm not convinced enough to bet on it. We'll simply have to...read and find out.