If I remember correctly, if you choose your bodyguard as the heir, and choose to uncover the truth, you will find out that your Sister is the traitor.

If you choose your sister as the heir, and choose to uncover the truth, you will find out that your Bodyguard is the traitor.

If you choose the scientist as the heir, and choose to uncover the truth, you will find out that your Sister is the traitor.

So, considering the background stories of the three characters, my personal understanding is, that your bodyguard and your sister conspired together to overthrow you, and the chief scientist is not involved.

I have heard from others that all three worked together to do so. But if that is the case, should not they "take turn" to be the one that is uncovered as the traitor, according to your selection as the heir?

All of them...they're all traitors. You can't trust anyone!!!

It's been a while since I played the United Empires campaign, but if I'm not mistaken Dimtri Lenko is one of the or the mastermind behind the rebellion. And I'm pretty sure his conspirators were his brother Hadri Lenko, and Lena Zelevas. I can't remember if the Scientist was also one, but then again I don't remember ever picking an option that resulted in her being one.