I am 5'5, 188lbs, and I rode HP and the Forbidden Journey last week with no problem at all. My stepdaughter is also heavy, but extremely busty also had no problems. I would say prepare for the worst but hope for the best.
For your Aspie niece, I would suggest talking to her about hidden mickeys. We have a huge range of "interesting" in our family as well, and all our spectrum relatives are amazingly good at finding hidden mickeys. I found that just suggesting a search was a great way to head off a meltdown in the lines... as soon as the build-up started, a quick "hey, look around this place, there's GOT to be a mickey hidden in here somewhere!" could cut the tantrum off mid-whine. There is a book they are selling for $12.95 in all the parks, but there are several websites that are FREE
I'd also suggest that you sign her up for Sorceror's of the Magic Kingdom in MK as soon as you go in the first time. If you bring the whole group in together, everyone can get a pack of cards, even if they don't intend to play the game. The kids (of all ages) will end up with lots of cards, and people are starting to trade them (like the pins, but free), which could be fun for them all.
Both the hidden mickeys and the sorcerors game can be an easy source for casual conversations in the lines, which is so hard for so many spectrum types.
For you epileptic nephew, I second everyone's recommendation to bring a stroller and get a stroller=wheelchair GAC. It made all the difference in the world for us, and you can get a similar provision at Universal, with no problems, though it seems to be less familiar to the employees. At MK you will have a sticker for the stroller, plus a card you carry. At Universal, it's just a card. We took our 8 year old all the way through the line for "forbidden journey" in the stroller while she slept off the effects of fit... then she woke up and really enjoyed the ride. We got a few curious glances over the course of the week, but nobody said anything or gave us "stinkeye".
Using a stroller for an older child is not uncommon even for healthy children at these parks... the parks are big and the kids get exhausted. While there are lots of places to rent from, I bet you can get by with a $20 unbrella stroller from any
walmart-type place... within two days you've saved money over a rental, and they are easy to fold down and take along. With the park strollers, you have to give them back when you leave for the day, but it's Soooo nice to be able to take it with you on the monorail/ferry, across the parking lot to that spot in Guam where you parked...
