Was Aaron (Harun) an infallible messenger during the time of his brother’s mission, or did he become so after…

Answer

In the name of Allah, be advised, my son, that every prophet, every messenger, and every Imam is infallible from birth until they join the highest companion (i.e., death). It is known that Aaron (Harun) died before Moses (peace be upon them), and that Aaron was not a messenger but only a prophet. Infallibility applies equally to prophets, messengers, and Imams. Allah knows best.