What a lovely sister you are!
For your tickets I really would buy either on the gate or as part of your package deal. I would not, if you are on a tight budget, risk the tickets being 'lost in the post' and TBH, buying on Ebay doesn't necessarily mean you WILL get them cheaper. The cheapest place to buy is at the park itself.
How old is your sister? Like you say it is pretty easy to get cheap flights ot Paris, but it could work out cheaper for you to actually get a package deal to stay in one of the cheaper Disney hotels (Santa Fe or Davy Crockett) rather than pay separately for a hotel and tickets. If your sister is under 7, she will go free as part of Disney's 15 year celebrations until the end of March 2008.
My own personal choice would be to stay in a Paris suburb rather than in the city itself. We always go in a caravan using Keycamps when we do Disney - the Paris International site is very good for getting to Disneyland as there is a train station a 15 minute walk from the site and the train pulls in ight outside the gate at Disney
I don't know much about hotels in Paris I'm afraid but I am sure you will loads of ideas form other AB'ers
Have fun, what ever you decide - you'll LOVE it !
ojx