You should just keep your 6.1 and get her some cheapo $20 sound card (OK, maybe not $20 cheapo, it is your sister). There are a few nice deals on card/speaker combos going on at CompUSA right now. If you are gonna let your HTS do all the work through the SP/DIF digital out and use passthrough, there's no point in spending all that dough on features you won't need/use. I'm using an Audigy 2 with the exact set-up you are looking to use, and I'm not even completely convinced I'm getting all the benefits of advanced EAX HD and CMSS2 and Dolby Digital EX etc. etc. I'm pretty sure the analog signals are just being pass through to my receiver, which then uses its own Algorithms for emulated 5.1 surround sound. Of course, I know when DTS/5.1 signals are being decoded, b/c the blue led on my Sony lights up, but thats only for DVDs and my XBox.
I may end up having to buy/try the analog outs (3 x mini to RCA Y adapters) and plug them into my 5.1 component inputs to see if there is a difference or not due to the Audigy2's advanced HD engine. My guess is that there will be a noticeable difference, as I think it will do emulated 5.1 better than my receiver. This sounds like a pain in the ass, but its really not. Creative's software has worked very well for me so far. You would only need to change the input on your receiver, uncheck SPDIF passthrough in CreativeAudioHQ, and uncheck digital output only in speaker settings.
Chiz