Does anyone find it odd that the word "selfish" is being thrown around on here? (from both sides!!)
I wouldn't call either side selfish.
The OP is not selfish for wanting to provide a great trip for her nieces. However, I personally, as a mother of 3, would not agree to it. I do NOT believe this makes me selfish, either!
There are several reasons I would not agree to it -- although 5 years ago I thought we'd NEVER be in the position to go, we are going at the end of the month. My kids have never been to WDW and I think I would regret having let them go a year or two ago when we are able to take them now. Waiting a year or two isn't going to kill them. Maybe they aren't in a financial position to go now, but who knows what road lies ahead.
I never went on any vacations with anyone other than my parents when I was little. My parents were very poor and the only vacations we went on were tagging along with my dad when he went out of town on business, which was not fancy at all. Those vacations were some of the best times of my life. Sometimes it's not only about the magic, but just being together with family. Yes, my dad was working some of the time, but they always made it fun.
Ok, 3rd reason - as I was reading the posts, my daughter got up from bed to go to the bathroom. while she was out here, i asked her if she would ever go on vacation with Aunt Bobbi (mentioning lots of different places, WDW being one of them) and she said, "would you be there?" and I said, 'no if you went, it would be you, tommy, and Aunt Bobbi". She said, "But I wouldn't want to go without you and dad!" Then I switched it up and asked about other relatives and same response.
So to OP - keep that in mind. The kids may not be comfortable with it themselves. If the parents say no, it may be because they know their children and don't think they would do well with it. OR, it could be that they want to experience it. Either way, it's ok. You certainly aren't selfish, but neither am I (or others like me) for saying no. (i know you didn't call us selfish, but i'm just sayin'). It's an individual parent's call. It's incredibly generous of you to think of them. But don't be disappointed if they say no.
i wouldn't expect you to offer to pay for the parents as others have suggested. I would simply ask them if you can take them. If they say no, respect that. If you decide to ask them to go with you and the kids, just explain that your finances allow for you to pay for the kids. They'll decide on their own if they can do that. And don't think of yourself as the 5th wheel! Actually, there will be rides the kids can't/don't want to go on. This way, there's always another adult to ride with!
Sorry this got so long...