Walking through a crowded city, the speaker feels completely alone. Over a hundred people pass by, yet the world seems to rush past without noticing them, and the speaker stays guarded, unable to trust anyone. The crowd makes the isolation sharper, not softer.
Everything shifts when the speaker's sister is there. Beside her, the speaker feels brave, even able to "rule the world," because she needs them to be strong. Even at night, when the people vanish and the streets turn lonely again, the two whisper on the train and sing until their voices hurt, then sing again. The mood is warm and close, almost protective.
The ending changes that. When the sister is gone, "the whispers settle in." The word is ambiguous, but it suggests worries or unwanted thoughts creeping into the silence. The poem leaves us with a quiet unease: the speaker's courage seems to depend on one person.