I think we can reliable state that they were both level 19 when they were raised.
We know there needs to be about 5 levels of strength difference for any significant deference between Aes Sedai. Merana's thoughts regarding Bera and Kiruna show us that this is about the point were weaker sister teeter between obedience and less specific deference. Level 14 to level 17 is enough for Bera/Kiruna to take control, but women of stronger personality can and do hold out (Felaana shows us this too), with only 3 levels difference. However just one more level down we see Kairen and Demira stay quiet and accept polite orders from Bera/Kiruna.
Now we can look at how Siuan clearly defers to Citalia who is a level 14 and can assume, given Siuans personality, there needs to be a fairly wide margin of strength difference for her to obey so readily. We also know from Moiraine that Duhara, if not for being Keeper, would need to defer to Moiraine. With Duhara placed at level 23 it seems natural that Duhara must be at least 4 levels below Moiraine for Moiraine to believe this.
Placing Moiraine and Siuan at level 19 works well for this and explains why Merean was enough stronger than Moiraine to defeat her rather easily, but still gives Moiraine and Siuan significant status when they were raised.
Good analysis, but isn't one level enough for deference? I know that the greater the gap the greater the deference but one level is also acceptable right?
But I digress...
I'm not sure if this is directly contested in NS, but I theorize that Moiraine was stronger than Siuan by the time they were raised. If it is directly quoted otherwise in NS then my theory is nonsense, so please let me know.
Both were 3 years as novice and 3 years as Accepted. We know that women take about 10 years to reach their strength but we also know that stronger women progress faster. The increase is usually linear and neither woman was forced. Therefore it seems prudent that both of them would have taken less than ten years to reach full strength. I hazard to guess that both had no more than two years left before they reached their full strength making them approximately 75% as strong as they would have been. Considering they both have an end strength of about 81, this means they would have been on a approximately 61.5 when they were raised, or level 26. This is a level consistent with a middling sister, and obviously much weaker than Cetalia, who was on the verge of punishment Siuan for her attitude.
However, Moiraine was an established channeler (wilder) when she was taken to the Tower, and this fact was hidden by the AS who discovered her. By the time she went to the Tower she could already eavesdrop with saidar. She had the spark manifest when she was 16 and she also went to the Tower at 16, so the distance is less than a year, meaning that she is at most a year stronger than Siuan. Therefore Moraine could be at 90% of her potential at best, or level 18. We also know that Moiraine did not think Cadsuane was so much stronger than she was - and at level 9 she was about 20% stronger than Moiraine at the time. At level 18 Moiraine is much stronger than Duhara and weaker than Cetalia and Elaida.
Therefore my theory is that upon being raised Moiraine was at 18 and Siuan at 26. Within 1-2 years both were at 13.
Fan of Lanfear