sorella

In a sentence

• La mia sorella maggiore studia medicina.

  My older sister is studying medicine.

  Here, 'sorella' is used in its basic meaning, 'sister', specifying it's the speaker's own older sister using 'mia' (my) and 'maggiore' (older).

• Ho incontrato la sorella di Luca al parco.

  I met Luca's sister at the park.

  In this sentence, 'sorella' refers to someone else's sister, in this case, Luca's, denoted by ‘di Luca’ (of Luca).

• La sorella d'Italia festeggia insieme ai suoi cittadini.

  Italy, as a sister, celebrates with its citizens.

  Here, 'sorella' is used metaphorically to symbolize a close and familial relationship to a country (Italy referred to as a sister) in a figurative sense.