This is the 961st most frequent Italian word.
sorella
"Sorella" means "sister" in English.
Here, 'sorella' is used in its basic meaning, 'sister', specifying it's the speaker's own older sister using 'mia' (my) and 'maggiore' (older).
La mia sorella maggiore studia medicina.
My older sister is studying medicine.
In this sentence, 'sorella' refers to someone else's sister, in this case, Luca's, denoted by ‘di Luca’ (of Luca).
Ho incontrato la sorella di Luca al parco.
I met Luca's sister at the park.
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.