Another perspective:
Alix was awkward, extremely shy, lacked social grace, had more difficulty learning Russian than her sister, and was fundamentally unprepared for life in Russia as Tsarina. She arrived, became a bride, and became Tsarina so quickly that there was no time to adjust before assuming her duties. Her social interactions were so awkward that she was made fun of.
Her sister, on the other hand, was, by all accounts, perfect. She was gorgeous and never looked out of place. She was graceful, articulate, much admired, and charming. She seemed to be able to everything well........and everyone loved her. Everyone. She could do not wrong.
If I was Alexandra, I would have harbored some resentment. Perhaps it was subconscious at first, but even so, there would have come a time when it would have begun to come to the surface. Given Alexandra's personality, that resentment had to have contributed to what happened next. If Ella began to try to coach or counsel her, or disagree with her in any waym no matter how loving, it could have been taken as an affront.
Can you see how, if Ella changed into a saintly, super-religious woman who swore off all of the luxuries that Alexandra had, this would have REALLY begun to be annoying. Now, even Ella's religiosity is better than Alexandra's! (in a manner of speaking, of course). Why, not only do the nobles love her, but now the common people love her. And Alexandra knew how the common people hated HER.......