Thanks, Prince Eddy!
I think that Maria was a very difficult child and since she loved her father she could not blame him for his disappearance so shifted her resentment to Ella. At the same time, their personalities were so opposed. To Ella, like her mother, duty went hand-in-hand with royal status. Paul had, in her eyes, neglected his duty and her antipathy towards him after that would have been perceived by any child. Maria always comes across as a sulky character and I am very suspicious of her account of the 'cold hard look' in Ella's eyes when Maria kissed the back of her neck - it was probably shock if anything! It must have been so difficult for Ella, since Serge had made all the decisions about the children, and I agree that Ella pushed Maria into marriage too quickly - but in Ella's favour, it must be said that at first Maria had been eager to marry and - again like her mother - Ella seemed quite intolerant of indecision and shilly-shallying. (And, it must be remembered that Ella was unwell at the time of Maria's engagement so any shortness of temper can be excused.) I do feel that it would have been better though, if after Serge's death, Maria had been encouraged to go to her father in Paris. Ella put her off the idea probably because she felt she had to honour Serge's wishes. (That's just what I think anway!)
I am also suspicious of Maria's accounts of Ella's unkindness and lack of affection because so many other children delighted in her company and she went out of her way to make them happy, give them treats, play games with them etc. etc.