Hi Giselle. I teach a college seminar about independent travel in Europe for first-timers, and you asked a very common question. Some of the answer is dependent on the way you like to travel. If you like fancy hotels, fine dining and being pampered, obviously it will cost a lot more than staying in hostels or pensiones, having "urban picnics" in parks, and exploring on your own. I always travel the latter way.
So, you will have to set your own budget. It's not hard: visit FareCompare.com to get an idea of how much airfares will be from your airport to European cities such as London, Frankfurt or Paris, which tend to be the least expensive points of entry into Europe.
Then visit www.RailEurope.com to learn all about the many railpasses available and see how much they cost. Eurailing is still the least expensive way to get around the continent. Pay special attention to the Selectpass, which limits your travel to just a few countries and just a few days out of two months. Also note the Saverpass option, which saves even more money when used by two or more people traveling together.
The nice thing is, you can spread out your expenses over several months. First buy your plane tickets. Save up more money, then buy your Eurailpass. Then continue to save for your daily expenses. I try to budget between $75 and $85 per day per person when planning my trips.
If this is your first trip, may I offer a suggested itinerary? Fly into London, take the Eurostar (Channel Tunnel train) to Paris, take the Thalys (high speed international express) to Koblenz (in the German Rheinland), then head to Munich, from Munich go to Salzburg, then head up to Prague. From Prague take one of Europe's inexpensive regional airlines back to London. (Visit www.whichbudget.com to see which cheap airlines serve the route between Prague and London.)
Good luck with your planning. I hope you have a wonderful trip!
PS: One more thing if I may: As a writer, may I offer a slight suggestion: you should say, "My sister and I are planning...," not "So me and my sister..." "So" is never needed, especially at the beginning of a sentence, and in a string of people, we always put ourselves last. :-)