Taim would have needed 13 BA and 13 Myrddraal to turn this group to the Shadow.. this sounds absurd that he had such resources until a Forsaken made them available to him at the BT very recently, to purge the place of Lightsiders on the eve of TG....

We don't even know for sure Siuan did send this group of sisters with a small army. She planned to and gave some orders to her Keeper, but not very long afterward she was deposed. Likely, further decisions about this group were still stalled in the Hall by then. Elaida later took her own, fruitless (might have worked better had she not tasked Alviarin to see to it...) measures to recapture Taim, and never mentions Siuan's efforts, as if they never actually went forward. If Siuan's group has had time to be chosen, assembled and to leave TV at all, it's likely they were recalled by Elaida. They were also to have a fairly large contingent of Tower Guards, which is nowhere to be seen and certainly not with the Borderlanders (we would have heard in Ethenielle's POV), suggesting strongly the 13 have absolutely nothing to do with sisters involved with Taim.

We know these 13 Aes Sedai with the Bordermen have come from different places and became "one group" of 13 by chance. Some flocked to Paitar's court shortly after the Tower Coup. The King of Arafel is well-known for his trust of Aes Sedai, and for favouring the Green (his beloved and favorite sister is a Green). It's hardly surprising sisters would seek refuge/counsel with his advisor or at that court most unlikely to be friendly to Elaida. Other sisters bumped into Tenobia months later.

We also know since TOM how the Bordermen got convinced to leave the north. The first piece of the puzzle (this was given to us by RJ) is that Hurin came back with the news for the King of Shienar that Rand was TDR. The second piece of the puzzle is all the troubles Rand caused in the south and the breaking of the Tower which convinced the rulers they had to act. The third and last piece of the puzzle is the prophecy they may have to kill the Dragon, kept secret for generations by the rulers of Arafel... That is what finished to convince the rulers it was their mission. This expedition has nothing to do with the Shadow. It may have been joined by a few DF, and certainly the Shadow has jumped on the opportunity to more or less take control of the defenses the rulers left behind (at least it was such in Saldaea), but that's all.

We know the rulers have acted secretely - discussing this at the top level. Most of their advisors were against the decision, which suggest their masters insisted... We know King Paitar only told his well-liked and trusted AS advisor after the fact, once all the decisions were made. His advisor had several sisters with her then, who flocked to her upon hearing of Elaida's orders to get back to TV - and who all decided to accompany the King's expedition (he thought he might need AS help down the line, and so didn't stop her). They are most likely Green Ajah or most of them probably are. The rest of the 13 consists of a small group of sisters who bumped into Tenobia by accident near the Saldaea border, much to their surprise. They seemed as eager as Tenobia herself that Elaida's envoy (who never found Tenobia and was then in Maradon) didn't hear they were with her... which very likely means they were the three Salidar ambassadors on their way to Tenobia.

Brandon's excitement about the question, if you ask me, is probably due to the fact he had forgotten about these sisters Siuan possibly (but not certainly) sent to recapture Taim, and was eager now to go look in the notes and find out what happened to those...

Personally, I doubt these sisters are MIA. I think Jordan had in mind Siuan never could actually send them - she was deposed before the group left, or that Alviarin/Elaida sent a fast messenger to recall the whole group later. Elaida didn't want it widely known that Taim had escaped.