Prior to their assignment to Ebou Dar, Teslyn and Joline were Sitters for the Red and Green in the Hall of the Tower. 

I doubt that they were of middling strength as the Tower hierarchy leans toward strength in the Power.

"The Aes Sedai we are talking about are not particularly strong"

Incorrect.

ACoS,Ch17 - Joline and Teslyn are the same strength and are stronger than Merilille.  Merilille is strong enough to make a Gateway for Traveling when she was acting as messenger between Elayne and the Borderlander Army.  By contrast, Vandene and Adelas are not considered weak in the power, but neither are strong enough to make a gateway. LoC, Ch44.  They are strong enough to pick up two women, (linked perhaps) and stuff them through a second story window when they are alomst mugged in the Rahad LoC, Ch52.

I know that strength in the power is something that is generally left rather vague in the story but I would think that this would certainly put Joline and Teslyn in the top half amongst Aes Sedai.  We don't see anything like that in the rest of the series.

I don't believe there is any way one Aes Sedai, which we know Mylen to have been prior to her capture, could shield two others, both strong enough to create a gateway, to say nothing of three. 

As for the strength of the person already shielded, if it were one person, what you say would be true.  But in this case, Mylen is weaving and holding three shields.  I think I recall a POV of Egwene's in the Stone of Tear stating that doing two things simultaneously with the power is a lot more than twice as hard as doing one.  So weaving three shields would be an order of magnitude more difficult.

I'm just not buying it.  It's a error or there is something about the Damane/Sul'dam link about which we have not yet been told.

Aviendha's statement actually makes my point.  The Sul'dam HAVE the ability, they just don't know how to use it.  They would not need to use it if the a'dam was ading it to the link for them.

So at the end of KoD, Furyk Karede shows up at Mat's camp with one Sul'dam, Melitene and one Damane, Mylen formerly the Aes Sedai Sheraine.

The three Aes Sedai with Mat, Joline, Edesina and Teslyn recognize her and demand she be freed. 

At that time, Joline gasps and all three are apparently shielded. 

One channeler being able to singlehandedly shield three opposing channelers seems to go against what we know about and have seen regarding shielding.  Nynaeve and Moghedien are about equal in strength (at that time) and duel for several minutes attempting to shield one another.

I checked the Encyclopeadia WOT to check my facts on that chapter but the incident is largely ignored so I am wondering if this is a mistake in the text.

Surely, three channelers, Joline, Edesina and Teslyn, would be enough to break the shield woven by one single Damane and then shield her in turn, enabling them to free her?  I know Teslyn and Edesina think Sheraine is too far gone to be freed, but would that be true if the collar actually came off whist she was surrounded by more Aes Sedai than Sul'dam? 

Could it be possible that as a Sul'dam grows more familiar with the power, she is actually adding her strength to that of the Damane? 

That's the only logical explanation I can come up with for Mylen being able to shield and hold Edesina, Joline and Teslyn.

There is something dark in Bashere's future.  Tenobia will have him beheaded for Treason against the crown.  After that, something will happen to her, likely killed in battle since she is so reckless.

And the Answer is found in the thoughts of Davram Bahsere in Crossroads of Twilight

He is considering Tenobia and why she has come south with a large force of Saldeans.

But he knew her; he had taught her to ride, watched her grow up, presented her the Broken Crown when she took the throne.

How it got broken?  Anyone's guess.

Sorry guys, I wasn't shouting, I don't work so good before my first cup of coffee.

AT THE BEGINNING OF ACOS, ONE OF THE BORDERLAND RULERS MENTIONS THAT TENOBIA IS NOT MARRIED SO SUCESSION FOR THE THRONE WOULD GO TO HER UNCLE (DAVRAM BASHERE0 OR HIS HEIR, FAILE. 

I DO NOT THINK A DETAILED EXPLANATION OF THE BROKEN CROWN HAS BEEN PRESENTED.

"Why Fain? Because the attackers are pretty obviously his corrupted Whitecloaks.

Your evidence for that being...?"

Who else refers to the Aes Sedai as "witches"  Fain hates Aes Sedai and wants to kill Rand himself.  I'd never given this much thought but the facts do seem to play out.