I am a spanish speaking creator, the reason why he calls him "little brother" is because of affection, in spanish it is "hermanito" diminutive of "hermano", as in japanese "onii-chan" is used only in the form of affection, I don't have much practice in english, so to translate I must resort to other tools, that's why the translation could look rough. But, in each update I always try to improve the translations, but as it is not my native language, there will always be flaws.
If you have any recommendation, I will pay attention to it :)