There are many websites with partial lists of passengers however first you should find the ship. To find a ship by date you can search the link below.
For information on your great grandfather's sisters (including their names and if they married or if they immigrated etc.)you can write to the registry office (Ufficio Anagrafe) in San Marco in Lamis and request a family certificate (Certificato di Stato di Famiglia) in the name of your gg grandfather. This document will list all the members of the family with dates of birth and marriage.
Information on this record can be found at:http://italgen.com/crri.htmhttp://www.arduini.net/tools/records.htm
Sample letters in Italian/English:http://www.girimonti.com/icc/resources/letters/index.htmlhttp://www.arduini.net/tools/frmltr01.htm
Address to send your request:
Comune di San Marco in Lamis
71014 San Marco in Lamis (Foggia)