This film is an allegory

This film is about one (yes, only one) girl/woman, who is fighting to leave her old life as a youth behind and finally take responsibility for her life and a possible future family, finally commit to something. The younger sister (the girl) is the more careless one, enjoying her life on a day to day basis, while the older sister (the woman) actually asks herself, what her life is all about (as far as I remember, she actually states exactly this), looking for sense and security. When the younger sister sacrifices herself to let the older one go free, this means just, that the single main character is finally leaving her careless youth behind and becomes a full adult.

The two sisters are just two sides of one personality.

There are also in my opinion some insinuations about the relation to and the reception of men (brutal beasts, who you can suck dry nonetheless, against love and shelter). Some people might regard this as an old-fashioned way of looking at women's relation to men, but this is only my interpretation of this movie, not an evaluation.