Right. Part of the reason I pointed it out was because I believed that people were making the mistake of taking this at face value, even though she literally says a few pages before that they are "distant" relatives.He calls her 'dear sister' which was most likely 'Onee-sama' (or perhaps Ane-ue) in original. This is often used to address someone out of respect, not because of relation.
It's part of the problem with the "must translate everything; must not leave any Japanese" mentally. No! No, better to leave some original words behind and give a brief explanation of what they mean than to translate them into the (common) English equivalent, which isn't how they are being used.
These words specifically are irksome. "Onee-sama," "oji-san" and so forth; especially oji-san being translated as uncle when they are referring to someone they aren't related to... It just shows ignorance to the language and culture, and I have literally seen it in "official translations"...