Hello,
what happened to Vladimir and Alexei were they close and why was Alexx III more close to Sergei when obviously different in character.
I think I read in either The Romanovs or Romanov Autumn that strains began between AIII and Vladimir a) because Vladimir had come very close to the throne and b) their wives didn't like each other at all.
When Nixa died, AIII was next in line. However, he was in love with (I can't spell her name) and contemplated renouncing his rights in order to marry her. This would've made Vladimir Czar after their father. (This had been done with Nicholas I's older brother Constantine who stepped aside). Apparently the thought of how close to power he'd come gnawed at Vladimir.
Minny and Miechen were very different personalities and both had their own cliques within St Petersburg society. This further alienated the brothers.
Also the decision by AIII to limit the title of Grand Duke and Grand Duchess to children & grandchildren of the Tsar didn't sit well with Vladimir. It meant that his grandchildren (who otherwise would've been 'Grands') would be 'just' Princes and Princesses. This loss of prestige would also be accompanied by loss of income that was due to Grand Dukes/Duchesses. While some accepted this well, the Vladimirs didn't.
Had they lived to see it, I doubt the irony of Cyril elevating his children (born Princesses and Prince) to the higher ranks, would've been lost on any of them.