I really like the premise here and I think we can read this novel as a progression from innocence toward experience. 

I think Quentin's interest in his sister is related to his idea of how to maintain innocence in her because he is uncomfortable with her sexuality and with ideas of being tainted. Family honor is something that Quentin is concerned with and is something that his father has mused upon at length. 

Also, Mrs. Compson is a nearly absent figure for the children. She has withdrawn. This means that Caddy becomes the sole mother figure, in terms of affection, for the children. She is both sister and mother to Quentin, Jason, and Benjy. 

In this view, the Oedipus complex becomes even more complex than usual as Quentin seeks to protect his sister from any lover that would be a rival for her affections, though she is his sister and not his mother. 

I think it is natural and normal for every boy to be interested in sexuality as he gets older.  A boy can wonder about his sister's sexuality without having sexual feelings for her.  Quentin, however, is obsessed with sisters and seems to think that liking any girl is wrong.