Pointing to a photograph, Rohan said: 'She is my mother's only daughter's daughter.' How is the girl related to Rohan?

Mother's only daughter = Rohan's sister (assuming Rohan is male; the only daughter of his mother). That sister's daughter = Rohan's niece. (If Rohan is female and is the only daughter, then 'daughter' would be the answer — but typically 'mother's only daughter' refers to someone else if the speaker is male.)