Replying...
Intro. You are married amd you have a young child. You and your wife usually call a 20 years old girl, Alice, to watch your child when you are not at home. Alice has an identical twin, Sarah, which sometimes substitutes her. And one of them seems pretty interested in you, even though she knows you are married and a father... but which one of them?

Alice the babysitter and her twin sister Sarah

@Marco