Romy Schneider is "Sissi: The Young Empress," Empress Elisabeth of Austria, in this 1956 film, the second of the popular trilogy. Schneider was nearly 18 at the time, and absolutely beautiful and charming in the role.
I need to say here, for anyone non-European or anyone who has not spent time in Austria, Germany, Hungary, etc., Sissi was the Princess Diana of her day and in fact, remains popular. Her face is on everything from candy wrappers to pins, pocket watches, necklaces - she's everywhere. She's been the subject of musicals, countless books, TV miniseries, plays, and films.
As others have pointed out, this is an idealized story of Sissi, with lots of facts left out as well as the more negative aspects of Sissi's personality - her anorexia, for one. Here, she is still in the honeymoon phase with her husband, the Emperor Franz Josef; and she is also anxious to help to build a good relationship with Hungary.
Sissi runs into problems when she becomes pregnant with her daughter Sophie, and her mother-in-law, convinced that Sissi is too young to be a good mother, takes the child from her in order to raise her. Angry that Franz will not stand up to his mother and instead, takes her side, Sissi leaves him and returns home to her family. It's pointed out to her that she has duties as a royal that must be fulfilled.
In actuality, Sissi's mother-in-law was worse to her than shown in the film. Also, by the time she becomes Queen of Hungary, she has three children but the film only speaks of one, Sophie, who by then is deceased.
These films are incredibly popular in Europe and I believe are shown at Christmas. The color is beautiful, the costumes and furnishings are gorgeous - these films are truly a treat for the eyes.
Read up on Sissi to get the real story, and enjoy these romanticized films for what they are: Sissi-lite.