The son could only talk to his father in the sister's absence? Despite the ostensible 'answer' given at the end that God exists in the form of love, the film is full of ambiguities, as all the characters' reliability remain in question till the end.
“Reality burst open.” “Give me some proof of God!” “Longing and denial. Trust and distrust. I don’t know if love is proof of God’s existence, or if love is God himself.” “For you, love and God are the same.” “That thought helps me in my emptiness and my dirty despair. Suddenly the emptiness turns into abundance, and despair into life.”