My wife and I make it a point of never missing AP when we visit the World. Of course, we usually stay at VWL, so that makes it easy!
First, the restaurant. With a little bit of searching online, you can find a few pics of the restaurant. It has an elegant, laid-back ambience, fitting for a Northwest-themed restaurant and lodge. If you can, try to get a window seat as it offers views outside the building toward the lake. Though it is a signature restaurant and the dress is listed as business casual, you will find the occasional parks attire clad diners. I have seen both the quintessential black cocktail dress and jeans on the same evening. However, for the most part, patrons wear nice slacks, shirts, dresses, and even blazers. You won't feel out of place dressing up, especially if you make a later ADR when the restaurant usually has fewer kids. We have taken our young daughter there before, and they do a great job of catering to them both with service and the kids' menu, but anymore we prefer to leave her at Cubs' Den and enjoy an adults night out.
Then there's the service. You will find the occasional negative comment about the service which is true about any restaurant, but by and large the reviews are highly complimentary of the servers. We have had great experiences there. Not only have our servers been diligent, but they are knowledgeable as well about such things as the wine list, wine pairings, and ingredients. On our first trip, my wife and I decided to do the wine flight wherein you choose three wines to be served with various courses of your meal. We were having a hard time deciding between a few, so our server brought out the bottles in question and gave us free tastes to better allow us to make a choice. That type of service is absolutely topnotch. . .and smart. She also treated our daughter with genuine friendliness, as well as treating us as valued patrons. Yes, she got a huge tip.
Of course, what good is ambience and service without having the food around which to build them? On that point, AP again delivers. Honestly, we haven't had everything on the menu because a) it changes slightly on occasion and b) we are so taken with our favorites. That being said, we've yet to have a bad meal there. Their top appetizer is undoubtedly the smoky portobello bisque--it's amazingly tasty and pairs well with any number of light red wines. They used to have venison spring rolls which were superb, though I believe those are no longer offered. Their frissee salad is also highly recommended.
Of course, their signature entree items are the cedar plank salmon and buffalo filet--both are excellent. If you happen to be at WDW after the first run of Copper River salmon in Alaska, then I'd definitely recommend the salmon, though the price usually increases since those salmon command heftier dollars. Regardless, the salmon dish is done very well--the fish is neither too dry (a common mistake in many restaurants) nor undercooked. It pairs very well with an Oregon pinot noir. My personal favorite is the buffalo. It is among the best I've had anywhere. Honestly, if you haven't tried buffalo, you owe it to yourself to try it. Were you blindfolded and give the meat to eat, I doubt you could tell the difference between it and a regular filet. It's not gamey at all, just tastier somehow. As another has mentioned, due to its leaner nature, anything other than medium will render the meat tough. I personally prefer medium rare, as even with medium you run the risk of the meat being overcooked. A heavier merlot, syrah, or cabernet should do well with this dish. (For the record, the wine list is commendable, made up of some of the Northwest's top wines.) There are other entree items which are very good, but these two are the best known and most well-loved, it seems.
Finally, there is dessert. The top dessert is likely the berry cobbler, though all the others are good. To top it off, AP actually serves good coffee. They brew it via a French press right at table side, and it's strong, rich, and aromatic. It's a great way to finish your meal. I hope this helps convince you that it would be a great place to take your sister. Bon apetit!
You will want to go back every time you return to Disney. 
