assuming the mystery child is the brother,he may want to hide his identity to Conan,and the fact that he's indeed a victim of APTX.
if he would have say "brother outside the domain",maybe Conan would make the link with the second brother.The child seems to be vey careful,and doesn't trust Conan yet so of course he could lie,in order to take less risks.
also the child seems to have health problems ,that could fit with masumi description(Akai is the strong one,and the middle brother is the smart one)