While the person answering below is right and both zusters en zussen can be used as plural for a sister, I would like to add a little to that.
Zusters is mainly the word used for nuns, whereby one nun is "een zuster".
So while the word "zusters" does exist, is plural, and is used informally(!) for saying you have sisters, if you want to be correct and prevent mix-ups, you really ought to say zussen. You would never write or say in proper spoken Dutch that you have "zusters" unless you somehow own a convent.