Board Thread:Wiki/@comment-30049797-20160924003216/@comment-24036697-20160924024227

Saltyhentai wrote: my friend, she wasn't adopted either. she is neither legally or blood related to the jeagers. she just chilled with them for a year. not even once did she refer to eren's parents as her own, or even take their last name. why is this fandom so ignorant. The real question is why do you feel the need to be so abrasive? Nobody here is attacking your opinion.

As GodKingReiss stated, the official Guidebook lists her as adopted, so it is canon. And just because we never saw them do any kind of legal paperwork in the series doesn't mean they didn't. It's just not relevant enough to the story to be shown (we also don't know how adoption is handled in the AOT universe; it could be completely different than in the real world).