Equally, there's no talking to people who don't bother to read their own faction's lore... I was referring to canonical material from Farsight supplement and to Lexicanum. What's your excuse for ignorance? How the fuck am I a zealot for correcting you?
Specifically:
http://wh40k.lexicanum.com/wiki/Tau
"Should an Ethereal order a Tau of a different caste to kill themselves, they would be obeyed instantly and without question."
Reference - 8f, Codex: Tau Empire (THE DEFINITION of your faction's canon).
Also:
http://wh40k.lexicanum.com/wiki/Ethereal
"Their power over the Tau is such that an Ethereal could tell a Tau to die and that Tau would do so quickly and gladly. The Adeptus Mechanicus would give much to learn the secret of the Ethereal's power. There is strong implication from the similarities between an organ possessed uniquely among Tau by the Ethereals, and an organ which Q'orl use in pheromonal communication that this ability may be due to deliberate engineering in the Ethereals' past."
And also.
http://wh40k.lexicanum.com/wiki/O%27Shovah
"After the incident on Arthas Moloch, Farsight returned to his established holdings on the far side of the Damocles Gulf. Farsight began to secretly become highly distrustful of the Ethereal Caste, coming to suspect that they may be controlling the Tau by more than simple charisma and wisdom, perhaps employing more sinister methods. In the aftermath of his victory, Farsight came to realise the threat of Chaos and the fact the universe was not the orderly rational place described in Tau philosophy. He also came to the realization that the Ethereals as a caste knew something of this but had kept it from the Tau.[4b] At first he merely thought their leadership was flawed, that the other castes would be better off seeking harmony with each other without their oversight. But the more he thought on the instances where obedience to the Etherals had been taken to illogical and foolish extremes. Worse, many of the actions taken under their guidance had seemed at the time both wise and necessary seemed now deplorable and wrong.[4b] In addition, Farsight began to live far beyond the normal life span of a Tau.[4h]"
And lastly you are wrong in thinking that tau are invisible to daemons. They are not a race of blanks, they simply have very weak souls. If a daemon was right in front of them, summoned into the Materium, it would very much notice them. Furthermore, Farsight fought daemons and they were the ones who slaughtered the ethereals accompanying him, freeing him from their control.