I did my best to keep the title spoiler-free, as this is a major story decision, but the rest of it will obviously have to have spoilers in it.
At the end of Samara's loyalty mission, if you have enough Paragon/Renegade points at the end of it, you can fight Morinth's sway over you. You then get a choice to either kill Samara or kill Morinth.
The default action for this choice is to kill Morinth, as, if you don't have enough Paragon/Renegade points, you won't be able to help in the fight.
However, if you choose to kill Samara, Morinth then takes her mother's place on the ship, which apparently fools everyone (except Kasumi).
Under Returning Characters though, I don't see Morinth listed on the wiki, but I'm unsure if this is only because she was not confirmed in press releases before release.
In what capacities do Samara and Morinth return in Mass Effect 3 (squadmates, mentioned by a NPC, quest-related, increases to Galactic Readiness, etc.)? I would like a comparison to help me make my decision about who I want to save.