A good change in rules is prohibiting people from buying with the purchasing price - i.e. every property is sold at auction. Best to also do a single round silent auction where the winner pays second place's bid (e.g. I bid 200, you bid 100, so I get it for 100).

With these rules you will quickly establish that some properties are not worth buying. These rules also have the benefit of making games ~30 minutes long because people run out of money extremely fast.

She might not be wrong. Some properties are landed on more frequently than others, and some give better rents for their purchase price then others. Properties and sets don't all have the same game value.

You obviously have to balance that though, can't just buy nothing.

Maybe your sister doesn't like playing and/or wants the game to end quickly.