An older brother is called a fratello maggiore whereas a younger brother is a fratello minore. Alternatively, you can say fratellone or fratellino respectively. A step brother would be fratellastro.
Valerio è mio fratello maggiore.
Valerio is my older brother.
The plural is fratelli, which means brothers but can also be used to mean siblings which indicates multiple brothers and sisters. Often though, you’ll hear Italians stating the exact number of brothers and sisters rather than using one word.
In quanti siete nella tua famiglia? – Siamo in quattro, due fratelli e due sorelle.
How many people are in your family? – There are four of us, two brothers and two sisters.
In the Christian religion, fratelli are all men, as they are sons of the same celestial father. You’ll hear expressions such as aiutare i propri fratelli or amare i propri fratelli (to help / to love your own brothers). Confratello means brother in the sense of a member of a religious community.
Fratello can be used for a friend you care for, or someone you’ve shared special moments with.
Luca è come un fratello per me.
Luca is like a brother to me.
You can use the noun as a friendly way of greeting someone, or to say goodbye.
Ciao fratello, mi raccomando, fatti sentire!
Goodbye my friend, make sure to keep in touch!
Finally, a beautiful word that derives from fratello is fratellanza: it can mean the bond between brothers, as well as the bond between friends or a group of people.