@furato @Tnyhy The Japanese language really screws translators over when it comes to "-in-law", "step-", and "Adopted because they all translate to the same word, "Giri" (or "Giri no" in practice). In some cases, there are contractions than use the syllable gi-, like 'gifu' for stepfather or father-in-law, but the problem remains. They translate the same Japanese word.
I believe that adoptive parents will generally use the normal 'son' and 'daughter' terms for adopted children, but adopted siblings often do not. There may be a contracted form for 'adopted little brother', but she may be calling him 'giri no itouto', which is the non-contracted version of "stepbrother, brother-in-law or adopted brother who is younger than you."
The translator may have decided that having them say 'adopted brother' and 'adopted sister' would sound weird. At least "brother-in-law" sounds natural in English, even if it is mis-describing the relationship.