I've been searching everywhere for this film, any help would be great! I remember so many details but still can't find it...
(tl;dr - Anime Brother and One-Armed Sister on Post-Apocalypse Floating Islands (with Ships and Guns and Clans at War) Exiled from Clan, Fall to Earth, find a community of Misfits on the Surface. Giant Robots/Monsters end up attacking the bad guys' main floating city (Activated on Purpose or on Accident?), and the surface misfits form an army and go to the floating islands, saving the main bad guy before she is killed).
It's an anime (slim chance just animated?) film set on floating islands above a mist-covered destroyed Earth. It's a hand-drawn anime style, reminded me of Nausicaä of the Valley of the Wind or something. I'm based in the US and I must've watched it online (maybe Crunchyroll?), in the last year or so (2018/19), though the animation style makes me feel it was produced much earlier. It was probably in Japanese/subbed English, though easily possible for it to have been in English.
The islands are peopled by a number of clans with varying levels of tech: steam-type aircraft, guns, but also more medieval stuff. The protagonists are a sister and her younger brother whose mother is killed when their clan is attacked by a more powerful "bad" group of soldiers. Their father with a big beard and berserker style goes mad with vengeance, obsessed with killing the group. The sister (who maybe has only one arm, one hand, or a gun-arm?) wants to prove herself, but the younger brother is basically a pacifist and loves to draw. Their clan goes go to raid this private island with a house and a weapons stash, a kind of computer, or maybe a bunch of monsters or a doomsday device? The father nearly tortures the guy who runs it. Turns out it's an ambush and the enemy army shows up.
The brother refuses to kill a guy and is exiled / rejected by his father, and when faced with a choice the sister goes with her brother. Leaving on a tiny ship, they get attacked by the bad guys and fall down past the mist to the surface of the Earth, which is war-torn and destroyed by giant robots or monsters or something? They find a community of misfits and exiles who live in peace with one another and decide to live there. The sister reveals some of her soft interior.
They go on some mission to an ancient underground technology bunker for some reason and either (1) get control of a powerful war machine/monster or (2) accidentally unleash it, and the beast/machine flies up to the islands and destroys the bad guys' main city/castle. The misfits form an army with the brother and sister to go up to the islands to fight (again, not sure if it was against the "bad" guys or with them to defeat the monsters). In the end, the leader of the bad clan is nearly killed, but is saved by one of the main characters, and there leads to some new understanding of peace in the society etc. etc.
I'm thinking that what the father got from the guy with the computer was a code or something to activate an ancient doomsday weapon which had earlier destroyed the Earth. This weapon is like a signal beacon which causes the robot/monster the brother and sister find on the surface to go crazy/activate and attack the enemy city. So if this is right, then the misfit army from the surface was probably actually DEFENDING the enemy city?
But this movie isn't mentioned in any of the asks I've seen, or on the various TV Tropes pages I would associate with it. It is not: Laputa, Last Exile, Skyland, Dragonhunters, Future Boy Conan, etc. Starting to believe I imagined it?
