An ordained priest is addressed as 'Father'.
A deacon (Priest's assistant, but doesn't have his powers),
seminarian (someone who study to become priest), religious brother,
are called 'Brother'.
A nun or religious sister, is called 'Sister'.
The Mother Superior of a convent is sometimes called 'Mother'. I'm
not too sure about that.
Of course, brother and sister are sometimes used to address friends
or people from church. Usually in a formal occasion.