@Akera93 It's a cultural thing.
In most asian cultures, calling someone 'brother' or 'sister' is used as a mark of closeness and/or respect, completely separate from actual familial relationships, and definitely does not imply a feeling of sibling relation. It doesn't translate super well into western terms.
In this, it's like how Japanese characters lose it with happiness when called 'onii-chan' or 'onee-chan' (big brother, big sister); MC is basically addressing ML affectionately, which is why he's ecstatic.