Piketty estimates that the average return to real estate worldwide is a little less than 5%.
In other words, the average property worth $6M would generate 6,000,000 x 0.05 / 12 = $25,000 in rent per month.
If we use the same assumption to calculate the value of your property, assuming an 80% utilization rate - $2000E/day x 365day/yr x 0.8 utilization / 0.05 = $12M.
So given these assumptions your sister was offered half of the value of the property. Of course this simple valuation hasn't been adjusted for taxes, depreciation, cost of maintenance, etc. but I doubt that would bring the value of the property down by 50%, and I'm willing to bet that you generate much higher than a 5% return on your properties if the rental mkt is as good as the OP has described.