The Austrian Railways site is here:
http://www.oebb.at/vip8/oebb/en/
That site is in English but the online booking facility takes you to pages in German. If your family members aren't confident with using a German language web page, they can use the UK site of Deutsche Bahn (which accepts bookings for most European Railways):
http://www.bahn.co.uk/db_uk/view/us/db_uk.shtm l
(Enter the travel details on the left. Follow the links through and it's possible to book, with tickets posted to a UK address, for �45 per ticket).
If they've got any queries, they can book by phone:
http://www.bahn.co.uk/db_uk/view/us/db_uk.shtm l
Chris
PS: The Deutsche Bahn site normally shows 'advance reservation compulsory' when this applies. It doesn't show up on trains on the Salzburg-Wien route, so it's likely that tickets can be purchased on the day.