I understand your points of wiew...Prince Lieven, Gdella, Nelly. In fact, I don´t like the image of Sissi forever young and always a legendary beauty, as a fairy queen ill-treated by her in-laws, specially the proverbial wicked mother-in-law. It´s not easy for me to explain in english, but I will try...and sorry for my mistakes!

1.-A young girl of fifteen raised up so far away from a court with strict etiquette as was the vienese court never should be married to an powerful emperor. Franz Joseph was madly in love with Elisabeth, but I believe that he never thought that his unexpected bride could not managed well the situation.
2.-I think that the struggle between Elisabeth and Sophia proves that the young empress was too inmature and unexperienced. Sophia was a pretty bavarian princess married against her will with Franz Karl, but when she convinced herself that the marriage was not a choice but a duty, she pronounced the famous words "I am determined to be happy, and I will be". I think that Sophia could provides a great support to her niece and daughter in law, but Sissi was so afraid of her aunt and mother in law that they never could had a benefit relationship.
3.-It was said that sexual relationship between Franz Joseph and Sissi was disastrous since the bad experience of the bride in the first nights after the marriage. I suppose it was a shock for a very young woman so naive. If a few years later he infected her with a veneral dissease, I think the question made a severe emotional damage to Elisabeth.
4.-A complicated point were the children. At the first, Elisabeth tried to raise up the children, but when the daughter Sophie died in Hungary, she shamed herself for this because her mother in law had advised her that it was not good idea to take the little Sophie and the little Gisela far away from home.