The Italian word for sister is sorella (plural: sorelle).
Ho due sorelle – una più grande e una più piccola.
I have two sisters – one older and one younger.
The official terms for older sister and younger sister are sorella maggiore and sorella minore respectively. However in speech it is much more common to hear sorellina for younger/baby/little sister, which is the diminutive form of sorella. The less common word sorellona (big sister) can only be used by a sibling – otherwise it would be interpreted as an insult (fat sister).
È nata la mia sorellina l’altro giorno!
My baby sister was born the other day!
You can use the word sorella to say that someone, such as a cousin or a friend, is like a sister to you.
Sei come una sorella per me!
You’re like a sister to me!
When referring to a half-sister or step-sister, the formal term is sorellastra, whereas adopted sister is sorella addotiva. Bear in mind that, as in English speaking countries, Italians may not feel the need to emphasise or clarify the blood relationship between themselves and their siblings by using very specific terms such as these. If the relationship is positive, they will often use sorella to describe any kind of sister – half, step, adopted or otherwise.
As in English, sorella can also mean sister or nun in the religious sense. A common synonym is suora.
Le sorelle stanno andando a messa.
The nuns are going to mass.
A sisterhood or sorority is known as a sorellanza in Italian.