Nebula rolls one of her eyes (the non-robotic one). 

"Only that it was obviously Gamora." She laughs bitterly. Though she's definitely still frightened by her father, her indignation obscures that fear significantly. "Consider: she and Cull are the only ones here that even like Grim? And Cull... isn't exactly the letter writing type."

Grim blinks, considers. "I've never known liking people to have anything to do with forming alliances." It's not a disagreement, just a statement.

The Sanctuary II fixes its course, making no further adjustments to bring it closer to inhabited systems. It can coast like this for as long as it takes Thanos to figure out this little puzzle.

He does not expect to be in any position to effectively helm the vessel throughout the remainder of the day, at least.

The other children get called in one by one.

Gamora looks at least as frightened as Nebula did.

"Letters? I... don't know anything about any letters."

Corvus doesn't look frightened at all.

"I've taken the liberty of putting together a compilation of surveillance footage from the residential wing that seemed relevant to the event in question. I expect it'll trivially exculpate me and, more importantly, might inform your assessment of all our testimonies."

Quizzical, nonchalant rumble.

Grim is surprisingly still, whisps of shadow wrapping around her, unsettled, but not in a way that indicates she's not used to such a...calm method of fact finding.

Calm fact finding has been Thanos' primary vocation for the past couple of millennia.

(The occasional brief, bloody planetary raids are just minor tangents of his ongoing pursuit of buried truths)

He regards his children, assembled before him in the forward viewroom with an indifferent starscape shimmering behind them. His gaze passes across each of them in turn: Gamora, Proxima, Cull, Grim...

Thanos has had long enough to mull this over, and has given the guilty party ample opportunity to confess of her own volition.

"Proxima. Grim. Hold Nebula down."

"What!?"

Nebula takes a single indignant step towards her father.

Before she can take a second one, Proxima has got hold of her from behind.

"Corvus. Remove her eye."

"Understood, Father."

Corvus deploys a multi-tool from his belt. His boots clank across floor grating--the only noise in the cavernous room, now, apart from his sister's stifled protests.

Grim obeys in time with Proxima, doesn't even look like she has to think about obeying.

Corvus whistles a merry tune as he unscrews the plating around Nebula's cybernetic eye. The block of machinery underneath pops out afterwards, a solid cube of cybernetic components that fits easily enough in Corvus' palm.

(The process of having about an eighth of her skull's volume removed from her face appears quite uncomfortable for Nebula, but not gory or torturous--her cybernetics were clearly designed with a certain degree of modularity in mind)

Corvus then pulls a retractable cable out of the datapad on his arm.

"Shall I initiate a wired link?"

Thanos nods.

"Fast scan of the past forty-eight hours of footage. Run the output through the projector. Slow it down to time-four speed when you get a pattern match for this document."

He passes Corvus the letter that Proxima found in Grim's room earlier that day.

Abject horror dawns on Nebula as (with her organic eye) she sees holographic video footage of the past-two-days-from-her-perspective playing out high speed in the air above her.

She hadn't known her optical upgrades had this function.

"I'm sorry!" She starts begging and blubbering before the recording even reaches anything incriminating. "It's not what it looks like... I'd never betray you. Father, I swear!"

The recording slows down. Plain as day in the hologram, a slender blue hand takes up the seditious note that kicked off this investigation, carries it discreetly down the hall, and then deposits it on the writing desk in Grim's quarters when no one else is watching.

Grim twitches, ever so slightly, at the idea of something that's part of her being used against her, but-

But she's also impassive. Her observations weren't wrong then.

Thanos strides up beside Corvus, so that he can look Nebula in the eye despite her current pinned-to-the-floor vantage point.

"Daughter." His tone of voice makes it clear that she'll regret it if her next words aren't spoken clearly and honestly. "Why?"

"Wanted to see if she'd take the bait."

Nebula squirms a little, turning to face Grim.

(Meeting eyes with her father fills her with fear, shame, and uncertainty. Meeting eyes with Grim just fills her with condescension. She finds the latter experience preferable)

Grim's face is still impassive, and she lifts an eyebrow, a silent challenge.

"Look. At. Me."

Thanos can tell that Nebula is speaking truth, now, but he can plainly see it isn't the whole truth.

"...and explain every line you wrote."

She remembers those lines.

She could say they were all lies, spun from whole cloth.

But that wouldn't be the truth.

And he'd know.

Somehow, he always knows in the end. That's how it's always been, ever since she was a little girl.

How can he see through her so easily? How can't he see through others that are so much more obvious? It isn't fair. It's never been fair.

          Run, while you still can

          Run, while you're still you

"I don't know who she was before you got your hands on her, but I knew she wouldn't be that person anymore by the time you were through. Knew you'd leave your mark on her--leave her changed, unrecognizable." Nebula tries to gesture at her own heavily modified body as she says this. Proxima still has a vicegrip on her, though, so her movements mostly just comes across as more squirming. "Figured that truth would likely to spook her, so it's what I led with."

          Don't trust him. Don't trust any of us.

          You might think he loves you now.

          He always smiles at the new ones most.

          But he'll grow bored with you in time.

          Escape, now, you won't get a better chance.

"I used to be your favorite, Father. You remember that, don't you? Before Gamora came along?" Nebula's tone manages to be simultaneously pleading and sinister. "And so now along comes Grim, ready to be the next link in that chain... yes, I told her your fondness for her would waver one day--and, yeah, I hoped that I could make that day come sooner."

          This is the last message you’ll get from me.

          If you’re too stupid to heed my warnings, fine.

          But please. Think about it.

          We’ll be making landfall on another planet, soon.

          He’s going to invite you along this time.

          There’ll be fighting. There’ll be chaos.

          There’ll be a chance for you to make your move.

 "She didn't respond to my warnings, though. Whether that was due to real loyalty or just cowardice I can't say for sure, but seeing as she never reported them to you I'd suspect the latter. I..." The last of Nebula's self-righteousness flees her. "I'm so sorry. All this trouble... my fault... and nothing good's come of it... I'm sorry..."

She doesn't have a way to respond, supposes it was cowardice, to not say anything for fear of losing the first thing she'd really wanted to be able to keep. (Doesn't know how to explain the burgeoning sense of comradeship she'd felt building with Cull, with Gamora, hell with Proxima, and the loyalty she felt to them that meant she felt the urge to cover for them, take whatever punishment they were due in their place...)

Proxima understands that look all too well. That burgeoning feeling that Grim can't quite explain? Nebula has lived with it for most of her life.

"You think I wasn't like you, at first?" She locks eyes with Grim again. "I tried to cover for her, took on shit that should've been hers for years. Look what it got me? Look! Look at me."

She's a truly pathetic sight: panicked, immobilized, her body as much clockwork as flesh and a big section of the former spilling out through the gaping cavity where the left side of her face ought to be.

"Every time he'd have us fight, he'd cut a piece off the loser. A foot, an arm... an eye? And so I'd always pull my punches, for all those years--piece by piece--so she'd never have to get cut on. I'd take the heat for her mistakes off the battlefield too: for My Perfect Sister, who never thanked me once."