I've just finished AP for the second time. I killed Sis and generally pissed off Albatross - I had "Animosity" or near that from him when this mission came up. Thorton tried to make Albatross track him down so that he'd send G22 troops to Graybox and you have the choice of trying to piss him off (and rub Sis' death on him) to do so. I tried, but perhaps it was not enough. Point is, you can get the mission if you are not liked by the guy, though I do not know the extent of what you can do with it.