Loneliness is hell? Is that he message of this movie?

No, this is not the message of this movie.
And yes, loneliness can be hell.

I think the message is that hell is others, just like Jean-Paul Sartre put it in his writings. L'enfer est les autres. The father's scandal not only destroyed his family, but also soured the relationship between the mother and the sisters. They avoid seeing each other, for that would bring back the unpleasant memories from their childhood.

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Hell is around us and inside us, just as heaven is.
Hell is hate, pain, distrust, fear, loneliness, revenge, deceit, cowardice. That's Hell to me, both the movie and Hell itself.