I'll be honest, as an Italian who watched this in my own language, this is better than many other similar films of the genre/theme. It's basically a perversion story about the aunt and I think this is enough you need to understand. The thing that strikes the most to me it's not the nephew who desire the aunt and doesn't even accept the occasion with another girl his age, but the aunt who is pretty pathetic in most scenes. She goes with every men she gets and after using the nephew do all her chores, he gets tired of her and goes with his own life.
She gets sad of not receiving attentions and wants him more than his husband, who actually is betraying her. But she can't make up her mind and still goes with the husband, which is basically most of today's female mentality predicted by a 1970 film. Honestly I don't know, it kinda makes her character pathetic.
A good message from it is, don't lose your chance with a girl because of your perversion, you'll end up having nothing. (unless the perversion works)
Shame for the actor who died at the age of 17, gotta respect a young guy who act a kiss with a woman of that caliber.