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. She filmed the scene in the midst of a production crew that was almost entirely Jewish, including her cousin Max, an actor, and also very modern and American (women with short hair! *gasp*) She did not tell her family what she had done because she knew that they would be upset and horrified about the whole thing, not just sleeve length!