Why I don't think Marluxia and Larxene are Seekers of Darkness
As the trailers show, Marluxia and Larxene are back. Most people are assuming that they are among the 13 seekers and have a piece of Xehanort in them. However, I don't think this is true. True, they were among the original Organization, but they planned on betraying Xemnas and taking control themselves. This shows that Xehanort is not always a good judge of character and can make mistakes. But Xehanort has shown that he does learn from his mistakes, hence why each new plan is more crafty and cunning then the previous one, and I bet his backup plans are put in place to ensure that if the current plan fails, he can try again.
However, this time around I don't think there will be any backup plans. Ansem and Xemnas are not as powerful as the ful Xehanort, so I don't think he can rely on turning into them again in order to defeat Sora. Sora is now much stronger then he was in KH1, and Xehanort knows this. And with any potential backup plans now used up, I have a good hunch that for all of Xehanort's cunning, doubt haunts him. Sora has defeated both Xehanort in his heartless and nobody form, and he is now stronger then he was at the start. In other words, Xehanort fears Sora.
And if Xehanort fears Sora, and this is is last chance to defeat Sora before Sora defeats him forever, then Xehanort will have to make doubly sure that any tool and plan he has for KH3 must be 100% reliable and have good odds of succeeding. And if I were Xehanort, the last thing I'd want are two pawns from the past who wanted to kill me. I know if they are members they would have a piece of Xehanort in themselves, but even the smallest odds of them going rouge is something to be concerned about, especially since to Xehanort, his next plan MUST succeed or else he won't come back from this defeat. Xehanort must rely on the other 12 seekers to be 100% treason proof.
As for what Marluxia and Larxenne are up to if they are not members I have no idea. It's plausible though that like Maleficent they are merely wild cards in the upcomming final battle with their own plans.