During my freshman year, we had a new member tea to which all the sororities' pledges/new members, presidents, and new member educators were invited. The idea was to allow us to meet new members in other sororities. It kind of backfired, though; people tended to stay in groups within their own sororities.
You could try something like that (either new members or everyone) with an icebreaker that would force people to interact with members of other sororities. For instance, you could make a list of people to find ("Find someone who plays a sport", "Find someone in your major", etc) but not allow people to use their own sisters to satisfy the list.
HTH
