'Kai, darling, big sister is really sad. It's my first time experiencing this in life.' 'Huh?' 'A moonlight flit.'
Literal
''Kai-chan [emphatic-よー], big-sister [topic-は] very sad [emphatic-よー]. Such thing [with-で] life-first experience is [emphatic-よ]'' ''Huh?'' ''Night-escape''
夜逃げ (よにげ) literally 'night-escape' is a specific term for 'sneaking away in the middle of the night to escape debt or other troubles' — a classic Japanese cultural trope featured in many old stories. The dots in 夜・逃・げ dramatize the word for effect. お姉ちゃん ('big sister') used as a self-reference by an adult woman can be a coy/cute self-designation.