My parents went last last in June on DL passes to PRG. I ended up routing them through VIE at the last minute and it worked out great. I had been tracking (stalking) the PRG, FRA and VIE flights and then booked them on the train to Prague from Venice. Easy transfer from VIE to the train station to Prague. My parents are very active and are in their 70's but managed fine. They stayed at the Grand and the Mandarin Oriental, both were great~The Grand would be a 3* and the MO is a 5*. The Mandarin had just opened up last year and there were great deals. Not sure if that is the case now.

My husband and I have also flown into FRA and then gone on via Lufthansa to PRG. The train would be an option for your sister. My husband spend a few nights in Prague and then went to Budapest, which I actually like more than Prague these days. Budapest is not on the Euro, so with the economy, the florent (sp.) is a great bargain. Budapest would be a better bargain than Vienna.

I've also taken the train from Prague to Venice and it is a beautiful trip. They could fly out of Venice.

Sounds like a great trip!