Boy, has a crucial basic detail about this film been misunderstood! To be absolutely clear: Méte and Belinda ARE related. They share the same biological father, but have different mothers, so they're half brother and sister. Not step siblings, as some reviewers on here have mistakenly concluded.
Admittedly it's a subtlety easily missed, particularly if you're watching this with subtitles as a non-Italian speaker. But no, contrary to what some reviewers have said, this isn't just Méte's father re-marrying another woman who has a daughter of her own; he's marrying the woman he's been having a long term relationship/affair with. The timeline, roughly, is that Méte's father left Méte's mother when Méte was young, and had a 20 year affair with Virna - the product of that relationship is his 17 year old daughter (and therefore Méte's half-sister), Belinda. This is spelled out in the dinner scene with Méte and Virna (Belinda's mother), where she says that she's loved Méte's father for 20 years, and that Belinda takes after her father. I think the part that confuses people is that Sergio and Virna are getting married, and so people assume it's a "step family", missing the fact that Belinda is Sergio's child as well. It's implied that after the 20 year affair, they're only now getting married because Méte's mother has died.
Anyway, sorry to go into such detail on this point, but so many reviewers on here have got this fundamental aspect wrong that I think the record really does need definitively correcting.
As for the film itself...it's an entertaining comedy drama, very stylishly shot and with an attractive cast. The taboo aspect adds an extra frisson of tension - and is definitely part of the film, despite some people thinking it isn't!
