Nicholas’ “sin” was losing to Japan in 1905, that set the wheels into motion.
The rest of it is complicated with Alexis, the only direct male heir to the throne and inheriting from Queen Victoria the genetic disease of hemophilia. And Alexxandra turning to Rasputin who had some occultic powers or suggestive or whatever it was. That became a source of hatred by many. Alexandra's sister was brutally shot in a pit. She had become a nun and was a martyr and saint, too.