This is what you must understand about Sirion:
Elwing is a young queen. She has strength and skill, but she has not grown into the fullness of them yet. She is so very young, especially by elven standards, so she leans on her councillors and everyone thinks it is probably for the best.
This is what you must understand about the councilors:
They are not, mostly, bad men. That does not mean they always do what is right.
There is one that was in the thick of fighting for Doriath. He barely escaped with his life. His memories of the ruthlessness of those he faced assure him that there is no chance the boy princes are still alive. Anyone who comes forwards claiming to be them is a liar and imposter, up to no good. (And if he cannot bear the idea that they might still be alive because that would mean he has failed them all this time by not saving them, then that is no one's business but his own.)
There is one who fought in the Nirnaeth. The night before the battle, he heard Maglor sing, and he heard Maedhros speak, and he is quite sure that no one could live with those two for two decades and not walk away wholly entranced. If the princes are still alive, they are all but bespelled and cannot be trusted. (And if he felt carried away by those shining words long ago, then that is just further proof that he is right.)
There is one from Gondolin who is still quite wary of all outsiders. (And if he is loyal to Lord Earendil above all else and has no wish to see him displaced by some unknown king, what of it?)
And if there is one or more who like the balance of power the way that it is and would not see it disturbed, there are others who have seen far too many tricks of the Enemy and who will not trust this one.
So when Elured and Elurin turn up even after they have refused to pay the ransom for them, perhaps they are right to be suspicious.
They are not sure they can trust Elwing to be. These may be (are) her brothers after all, and they are not at all sure what she will do if she sees them.
They miss their opportunity to turn them away at the gates, but one of the councillors manages to catch them before they can get an audience with Elwing. He invites them in to eat. To talk. To get their bearings.
He weighs them up, and he does not like what he sees.
Red cloaks as if they are Feanorians. (Cloth is not easy to get. The cloaks were remade for them from some of Celegorm and Curufin's old clothes. The brothers do not need them anymore, after all.)
Determined confidence that is not easy to steer onto a more palatable path. (Elured and Elurin value council, but this councillor and those of his fellows he has invited over are not like what they are used to. Those who rose to power in cautious, careful Gondolin and Doriath are not like those who follow the star of Feanor.)
And quiet songs of power ripple under their every word, waiting to strike. (They are grandsons of Luthien, conceived and born under the light of a Silmaril, raised under the tutelage of one of the greatest bards to ever live on the very edge of what is defensible territory. They are powerful, and they have not been taught to hide it. If it comes to a fight of song or power, they will win.)
They are dangerous, and they are counseling that the Silmaril be given up. They are not trustworthy, and they cannot be cannot throw them out of the city without a fuss, and they cannot afford a fuss. They cannot kill them without becoming kinslayers, and they do not want to think of themselves as kinslayers.
They do the only thing they can think of and drug the meal.
The twins are used to danger, but this is not what they are trained to watch for. They are wary of dead plants at the edges of poisoned pools, of crushed twigs that mean orcs have passed, of dark looks that mean someone would like to take a sparring session too far while dropping confusing hints about Doriath. This? This, they were never trained for.
Elured is less hungry and thus eats less and succumbs slower. He sees his brother fall, and he leaps to his feet, a song ready.
Three of the councillors fall asleep before Elured, too, succumbs.
They lock them up. They don't see what other choice they have. They leave more drugged food and water while the twins are sleeping. This forms the pattern.
They do not feel good about this, most of them. They just do not know what else to do.
Elured and Elurin resist the water for as long as they can. They plot. They try to break out.
They are weary and heartsick and still feeling the lingering effects of the drugs. They fail.
They talk, during the attempts. They wonder if their sister knows. They wonder why this has happened. They wonder if the Feanorians know where they are, and if they will come to get them when they figure it out.
By the third round of drugs, Elurin is unabashedly hoping they do, and that they burn down the city while their at it. Elured is less certain, but he is sick and thirsty, and he wants badly to go home. He is not quite sure what home is - a fuzzy memory of a warm nursery and his mother's soft embrace, or a warm hall with fierce laughter ringing around a dinner table half filled with those who need no food - and it doesn't matter. Both are impossibly far away.
As it happens, the sons of Feanor do know where they are. They have spies in the city, and they know that the twins safely arrived.
The spies have no more to report than this.
This … concerns them.
The idea of attacking Sirion and facing the boys they raised is abhorrent. The thought of leaving their father to suffer for yet longer is equally so.
And they are increasingly concerned that the twins might be in trouble.
The Oath - the Oath that they've been fighting and denying for twenty years by not seeking its object more diligently - objects. It hits the dead the hardest, causing agony to their unshielded forms.
Maedhros gives in. They will attack the city, though it barely rates the name. At this point, what does it matter? Kinslayers twice over or thrice over, they are condemned all the same.
He gives orders that Elured and Elurin are to be allowed to escape or taken alive. He will not see them dead.
They attack.
A few of their people desert, but not many. The torment of their dead lords increases their loyalty to a fanaticism, and many of those with softer hearts are worried for the twins and have managed to convince themselves that this is in their interests.
None of this changes the fact that most of the brothers look at what they have wrought and hate themselves a little more.
The dead cannot fight, but they can scout. Celegorm and Curufin hunt for the Silmaril through the city. They lead Maglor and Maedhros to Elwing, who throws herself out of a slightly lower window than she otherwise might have reached. This changes nothing.
Caranthir does not make it that far. He is cut down in the streets. Het gets up and keeps going.
Amrod and Amras fight together, the dead looking out for the living. They are looking for the other set of twins.
But somewhere in the chaos, a fire has been set or spread out of control. They find the twins in Sirion's little used prison, the guards bribed to keep their silence until Amras sends the guards to a silence far beyond bribes.
But the smoke finds them first.
Elured and Elurin are awake but weak, pressed desperately to the floor to avoid the smoke, trying to sing through parched throats to ward the approaching fire away. Amrod tries to encourage them while Amras desperately tries to pick the lock, ignoring the way the hot metal stings his skin.
The door finally creaks open. The building is on the verge of collapsing around them. They are running out of time to escape, but Elurin is starting to succumb to the smoke. He cannot get out on his own.
Elured is weakening too, but he tells Amras he's fine, to take Elurin.
He is not fine. Amrod, the only one not bothered by smoke inhalation, knows this, but there aren't any better options. They have to try.
Amras gets Elurin out. He turns around, and neither his twin nor Elurin's is behind him.
He goes back in.
Elured has collapsed. Amras is desperately trying to wake him up. Amrod grabs him and tries to drag him out.
That's when a burning beam in the ceiling falls and traps them both, catching their clothes alight.
Amrod should flee for help, but for a moment he's stunned into stillness and then he tries to help himself before remembering he can't and going.
Amras's scream tells him he's probably already too late.
He is. For both of them. His men come back to a collapsed building. Elurin is just outside the zone of destruction, still hanging onto life by a thread. Amrod can hear his brother's mourning wail inside a place no living man could be.
He goes to join him.
Maglor finds Elrond and Elros before he knows about his brothers or about Elured. He almost wants to laugh, in a hysterical sort of way, but the city is on fire, and the symmetry is there, and what else are they supposed to do?
Besides, they're both crying, even if they're trying to hide it, and they need help, and Maglor is there, and this, at least, is right, so he helps. To prove something, maybe, or even just because of that. Because it's right. Because they're children who have never hurt anyone. Because they're family, in more than one sense, and as he tells Maedhros what else are we supposed to do?
Maedhros also doesn't know yet, and while he's a little afraid Maglor is trying to replace their missing twins, he's also accustomed to the idea of taking children as dubiously useful hostages by now, so he doesn't protest.
Then they find out.
There is still no question of leaving the new twins behind because there is still nothing else to be done, but Maglor is the only one who can bear to have much to do with them. He sets them in the cart with Elurin in the hopes that the three of them can keep each other from fading and summons up all the power his scratched throat can manage to sing them into a tighter hold hold on life.