This can be done, but it's trickier than you've described. The main problem is that receivers lack a "main in" connection directly to their amps. Instead, you will need a receiver with a multichannel analog input, at which point you will connect the pre-amp outputs from the 950 to that input. You will then need to find a volume setting on the receiver that works for calibration (I'd probably set the center channel trim on the 950 to 0dB or maybe +5dB and then adjust the receiver volume until you get 75dB with the 950's test tone), then calibrate the 950. After that, your sister will need to never change the receiver's input or volume setting - altering either will wreck things. The receiver will also not have a trigger input to allow it to be turned on automatically by the 950, so she'll need to remember to turn it on each time or you'll need to set up a universal remote with a macro that does that for her.

A used amp may cost slightly more, but it'll be easier to operate and will likely sound better - receivers in this price range aren't going to have especially good amp sections in them.