I don't think Rebecca's family was orthodox. (Her grandparents maybe, but I'm pretty sure her parents were not)
Pretty much every Ashkenazi European Jew in 1914 was what we would consider "Orthodox" today in their observance. Rebecca wore this dress for a movie scene as a costume, without her parent's knowledge or permission.
Thanks for the clarification about the dress. I A girl who has dressed modestly all her life and has learned the Jewish laws of modesty from a young age would not suddenly be comfortable walking around with a low neckline and bare arms. It's not just about parental control.