She doesn't move immediately, but says quietly, with conviction, "I've never known him to break his word. Whatever promises he's made you, he'll do everything in his power to keep. Even if it breaks him."

Maybe she shouldn't have added that last part. Getting killed in any way that sticks here apparently isn't a concern. Emotional and mental toll, however-

Koschei is entirely capable of backing himself into a corner where he disregards his own limits and boundaries. Amata knows this from experience. Bianca, as an inmate, may be the only with less nominal power than Koschei, here, but Amata's sure she has emotional power over him, nonetheless. So far Bianca doesn't seem inclined to egregiously abuse it. Amata can only hope that holds.

That moment of seriousness passed, Amata says amiably, "Sure thing." And reaches for the bottle as asked.