They're actually not adoptive siblings without the patch. Check for where it mentions adoption. It doesn't mention it once. They are just staying at Noji's house. There has to be a formal explanation, but we're not presented with one. Simplest one that avoids any terms being broken is Noji runs a foster home or house for troubled youths. Such things exist, but the game doesn't explicitly say. The official game allows you to fill the gaps with what your mind finds plausible.
Some people argue that Noji is their Biological mother because they really want that, despite it being massively implausible. The takeaway? Play with your own understanding. As long as it makes sense, it is your canon. The game has its official version of the script and it considers that issue unimportant to clarify.