"Ik heb twee zussen." is informeel en "Ik heb twee zusters." is meer formeel.

"Zussen" is common, correct and should therefore be used nowadays. "Zusters" is also correct and can mean sisters too but it is a rather old-fashioned word. There is for example a old song called: "dan moet je mijn zuster zien" which could be translated into: "Just look at my sister". In that song the female singer complains that all men look at her sister because of her beauty but completely ignore the singer herself.

and zuster isn't formal at all (i forgot to mention that) :p

In west-flemish we say 'zusters' for sisters but in other parts of the country and in Holland it means 'nuns' :) but in Dutch it's 'zussen'.

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.