Jan/Feb is a great time to go. Lower crowds for sure. Temps will be on the cool side, but that's okay - better than heat and humidity in some ways. Split stay between AKL and Poly will be awesome! Trip with siblings will be great fun. I've done several trips with siblings (and all our kids). My tips, and it sounds like you have done your research and have made some great plans:
1. Stop and smell the roses. I know that's a trite phrase, but I'm serious. Go in with the mindset that you're going to have a great time, make memories, and make your own version of a great trip. There is no "perfect" trip, but with the right mindset, you'll have as close to a perfect trip as possible and later on, you'll find yourself laughing about the little things that didn't go quite the way you had planned. Enjoy the little moments. Sit on a bench with some ice cream, watch street performers (like the Dapper Dans), take some rest breaks and chill out with a snack, look around at the shops, watch the ducks. Definitely ride a ride that you think looks "lame" (i.e. People Mover at night). You might like it.
2. Don't be held hostage by your plan. Don't be afraid to go "off plan" depending on what suits you in the moment. Know going in that your plan is probably overly ambitious and don't hesitate or fret about making cuts. Don't be afraid to improvise. If an ADR time/place doesn't work for you at a particular moment, remember that you can cancel 2 hrs in advance and choose something else. (If you're within the 2 hour window, modify it to a later day and then cancel it). There is tons of restaurant availability day of. I never book ADRs. You mentioned concerns about prioritizing things. I personally use
Genie+ and find it helpful. (I also personally refuse to stand in any line for more than 20 min). In any case, when in the parks, I find it helpful to have 2-3 "must do" priorities and be flexible with the rest. Don't get bummed out if you have skip something. One time recently, my teen and early 20's daughters were super hot and decided to spend 30 min splashing around at the little kids splash station by Dumbo. Then they rode little kid rides Barnstormer and Dumbo. That hour or so was not in my plan, trust me. But I sat on a bench and watched them and took a video. It was unexpected, hilarious and fun.
3. When traveling with siblings (for me anyway), I've found it helpful to have a "no forced fun" rule. Sometimes someone doesn't want to go with "the plan"... which is so annoying, because I make awesome plans - LOL / just kidding. Anyway, I go in with the idea that anyone can skip any segment of "the plan" at any time and meet up later. That seems to work. Otherwise, you as "the planner" spend the whole time feeling like you're trying to herd cats. It's no fun to drag along a reluctant participant. One Negative Nelly can put a damper on the fun for everyone. So what if one person wants to prioritize sleep and skip Early Entry? No big deal.
4. The Poly is one of my favorite resorts. Set aside some time to enjoy it. Take an afternoon to hang out at the pool. Get a Dole Whip at Pineapple Lanai (the outdoor ice cream window by the lobby). If you're around in the afternoon (usually before 3pm), go get in line at Trader Sam's Grog Grotto, by the indoor entrance. They usually open at 3. I get in line at about 2:30 and am given a number. They text when they open and my table's ready. Go in and have a drink. It's fun. (I don't drink alcohol, but get the fruit punch and it's still fun). Eat a light dinner/snacks at Tambu Lounge upstairs by Ohana. They serve some of the same food and it's fun to sit there at the bar. Rent a pontoon boat and tool around Seven Seas Lagoon. I have no tips for AKL. I stayed there only once (low price) and disliked it. Simply put, I didn't like having to bus everywhere and I found it too huge, too brown and generally not to my aesthetic taste. Would much rather have stayed at Wilderness Lodge (so pretty!) or Yacht/Beach, or the Swan/Dolphin, where I could walk to Epcot/DHS. I know a lot of people love AKL though, so take my opinion as one person's point of view only.
Anyway, sorry for such a long note. You've done your research, you've got some great ideas. Just go, relax, and you'll have an awesome time!