Deceptive love = theme of the film

It doesn't make any sense to you because you are viewing the middle daughter as the victim, when in fact -- as the movie makes clear -- she is every bit as responsible for the consequences of her having multiple boyfriends (unknown to each other) as a given boyfriend is responsible for his having more than one girlfriend (unknown to each other).

If you had more than one boyfriend, would you tell each of them about the others? Or would you consider that your private business, and none of theirs? You would consider it none of their business.

But if at the same time you discovered that one of your boyfriends had other girlfriends you didn't know about -- it being his private business -- you'd consider that boyfriend as being deceitful.

In short: check out YOUR double-standards.

And your obsession with deceit in personal relationships.

I also think it obvious you're projecting your own experience onto the film, in order to find deceit where there isn't any. The privacy of individuals about their "love" isn't necessarily deceptive -- whose business is it? -- and does not transform the "love" into being "deceptive love". The middle daughter -- who you insist on seeing as "victim" -- doesn't tell any of her boyfriends that she has other boyfriends/bed partners.

Nor, or course, do they tell her she isn't their only girlfriend/bed partner. It's when she stops by one of her boyfriend's place "needing" to talk that she discovers he can't because one of his other girlfriends is visiting. But she isn't a "victim" there either, because she's all along been doing the same thing. That is, though, where she finds the "free love" shoe on her other foot.