When hubby and I got married, we paid for most of it ourselves. I had been out of college and he was just graduating. We did not have a lot of money by any means, but neither did our parents. We had a nice wedding (not lavish) with about 100 guests. We found ways to keep costs down (heavy hors d'oeuvres instead of a sit-down dinner). My parents paid for my dress and gave us some cash (about $1500, I think), while his parents took care of our honeymoon and hosted the rehearsal dinner at their house.
A little bit in your sister's defense, I know even in trying to keep costs down, our wedding was about $6000-7000 (including the contributions from my parents) and that was 13 years ago. This many years later, I don't think $10,000 is that much for a wedding, reception, dress and all that. I am just basing that on what we paid and given the passage of time, so I really don't know. However, just expecting to have that amount of money handed to you is a bit much.