So... She was banging her uncle; who also banged her mom?
Obviously, it was never explicitly stated and its obvious that he really wanted to bang his niece. But to me its HEAVILY implied, perhaps even flat out stated without actually having the characters say it.
-Her statements about his hands being dirty
-when he's drunk he tries to come in her room violently but the door is locked so the knob just... well you know if you're reading this.
(its later repeated later when her bf shows up- she is dancing making you think its the uncle).
It seems while never actually clearly stated, its hard to say 'no'. Especially with how much hatred she has towards the uncle when we see them present day and she doesn't even want to be near him.
Also, it seemed he even had an affair with her mother(who I'm assuming was his sister-in-law). That monologue he has while sitting at the piano... its more then just a man remembering. Even the way its shot, its so intimate its like Bergman is trying so hard for us to read between the lines.
I see it as the Uncle has an affair with her mom. Then molested the daughter- who was too young to understand but as she got older and fell in love with a boy, eventually realized how she was raped. It seems like subtext to the character arc, and in the 1950s would not of been tolerated in a film. I more then anything wonder what the BF's diary stated about their relationship, since he seemed to be jealous of her and the uncle.