(Thanks to whomever combined my posts. :-) I just was thinking I should have done that.)
I don't have the book here, but didn't the sisters say that they have always existed within the same body? Just that during particular times of the year, one personality or another would be transcendent and in control? Perry mentioned that this explained why they were never seen together. I think they also said that the Morrigan had done something to suppress them and maintain control all the time.
As to whether or not we can trust them, that is a whole other kettle of fish... or birds... I think that perhaps the debt that they owed to Perry is now paid. She released them and helped them regain control of the body, and they helped her escape Nereus. My guess is that they are now waiting to see what happens next and will declare their "side" once thing has advanced more. I get the feeling that their self-interest will motivate them more than anything else. Although revenge will also be high up on their list. They didn't seem overly pleased that Morrigan had been following another's orders, lol.
Some additional things that popped up as I was doing research (warning the history geek has been unleashed, lol)...
1. Nereus (The Old Man of the Sea) is the son of Gaea (Mother Earth, who I also found referred to as a Titan, or as the mother of Titans) and he married a Titan (Doris). Their union produced the Nereids, including Amphitrite with whom Poseidon (or Neptune in Roman mythology) fell in love and eventually woed and married. If I have my hierarchy right (and please correct me if I am wrong) this could make Nereus a first generation Elder, on the same generational level as Hekate and Mars. Which to me means that if he is repaying a debt to Dee's Elder by attacking the Sorceress, then Dee's Elder is either an incredibly powerful first generation Elder or perhaps a Great Elder who survived the fall of Danu Tallis. Hmmmm more research is definitely needed, lol.
2. In looking for twin gods/goddesses with dual male and female aspects I came upon some interesting information regarding Celtic lore and The Morrigan. I apologize if this is a repeat of things that have been posted elsewhere in this forum. I haven't read all of the threads all the way through yet. I don't want to post this somewhere else either as it pertains to the aspect of The Morrigan that becomes clearer in this book.
In Celtic mythology the Tuatha Dé Danann (People of the Goddess Danu), are one of the mythical races said to have settled in Ireland. The Dannans are seen as deities as well as heroes. They are also seen as immortal in that they could live for a long period of time, but could be killed. In this tradition, The Morrigan is described as a tripartite goddess, a goddess with three separate personalities or aspects. She is either referred to as three goddesses with the same name, or as one goddess with three aspects in one body.
To me this also brings to mind Danu Tallis, and ther perhaps some Elders fled first to Ireland after the Island sunk bringing forth the mythology of Danu, the mother Goddess.