Source: The Great Interpretation of Dreams (Ibn Sirin)
A laughing (smiling) sister in a dream indicates the arrival of an absent person or joy and relief.
Source: Perfuming Humanity in the Expression of Dreams (Al-Nabulsi)
A laughing sister indicates goodness and benefit.
Source: Signs in the Science of Expressions (Ibn Shaheen)
A laughing sister is interpreted as goodness and joy.