Honestly, I'd keep the ADR at Captain's Grille. It's considered in recent years to be a hidden gem in the BWI/YC/BC, area and based on that reputation, my extended family booked a latish ADR (8:30 pm) there last November. Our meal was fantastic, one of our highlights (only California Grill was better) Everything we had was top notch, not just the food but including wine and mixed drinks. Despite it being the late part of the dinner shift, service was excellent as well. The crowd in the dining room went from full to almost empty while we were there, but we never felt rushed to leave, and our server was extremely attentive to our needs. We would eat there again without a second thought, even in their temporary space in BC. Unless the chef or wait staff has changed, I can't see a temporary move causing the quality of food or service to decline since last December. Perhaps a look on Yelp or Tripadvisor would help find more recent reviews. But we found the food there excellent (and, as we were staying at BC, having such a quality restaurant so close to our resort the night half of our party arrived was a nice option).
We ate at the Fountain the same trip, at about the same time...8:30 pm. After experiencing both, I would never send anyone to the Fountain over Captain's Grille (except maybe my worse enemy). The food at the Fountain was ok...at best. We mostly had burgers, and frankly, they were about the quality of QS burgers but at a higher price. Drinks were ok...when they finally came out. Shakes were very good, so maybe it's worth going there to get ice cream. But what made this a "never again" place for us was the abysmal service. It was the worst we've ever experienced at a WDW area restaurant (think, waiting for almost half an hour for
half of the table's drinks to arrive...in a restaurant over half empty...with no explanation or apology whatsoever for the delays that continued throughout our meal). Our party
wanted to spend money there, but the slow service turned us off (a few in our party actually left to find grab and go food). It was a stark difference to our experience at CG.
Also, maybe your sister looked at the menus of places near BWI when deciding on an arrival night ADR and just decided she liked the look of the one at CG

. My siblings travel with me to WDW a lot, but they travel on 'solo' trips (i.e., with just their family rather than extended) as well...and unless they ask of my advice (as I'm considered the family Disney expert), I tend to ignore their ADR plans.