Wow guys, I am really blown away by all of the comments.
Thank you so much for providing your input and helping me w/ this.
I'd also like to say thanks for those who defended me and my motives. Like many of you, if I took these children to wdw it would be about them and not me. I'm thinking that if the parents do say no, maybe they wouldn't mind if the girls just came to visit or if we did different theme parks other than Disney.
Please if you have beef w/ someone on this thread, I would greatly appreciate it if you would private message among yourselves and not hijack this thread. Let's keep it nice here.
Okay, so just to address something - I am not so concerned about taking my nieces for their "1st time." I know that the first time is significant for a lot of people, so that's why I mentioned that they had never been. Honest to goodness for me this trip is more about me spending time w/ my nieces, not "experiencing their 1st time at wdw."
And certainly I am not trying to steal that from their parents, I just think that if they are unable to take them (even though they would like to) this is something that I could do for them.
Also, I can tell that this is an emotional subject so I would never bring it up to my nieces before I ask their parents, nor would I make their parents feel bad or inadequate for not being able to take them.
And my brother has been to wdw a few times when we were younger, the last time he went was about 12 years ago. His wife has never been.
Clearly, the parents want to take their kids to DW the first time. The OP says it right there. And I have clearly said that I would rather MY child's first trip down Main Street be with me and my wife, which it was and it was AWESOME!!!!! The OP says her sister is planning to take the kids, but not soon enough for the OP.
The OP's dream, IMHO, takes a back seat to the parents' wishes. That very, very, very first walk down Main Street is a sacred moment. Just like Christmas morning.
Again, this is not about me taking the unique experience from the parents, I just thought it would be nice to spend some time w/ my nieces in wdw. I would offer to take them had they been to wdw already & their folks couldn't afford to take them again.
And just to be clear,
wanting to take their family and
being able to take their family are 2 different things.

I think if the OP wants to experience the magic of taking a child to Disney for the first time, she should have a child of her own to take.
Again, I am not wanting to experience the magic of taking a child to Disney for the first time, I just want to spend some fun time w/ my nieces.
Also, having a child just so you can experience the magic of taking them to Disney sounds pretty silly to me.
I don't get this at ALL. There's no rule that everything has to be equal and fair between siblings...especially adult siblings. Just because you offer something to one doesn't mean you have to offer it to another. I really don't see how this is an issue at all. I mean - we have a close family but man...we're not all up in each other's business to even know or care who gives what to whom and that kind of thing. It's really none of anyone's business. If my sis gave something amazing to my other sis, I'd be happy for them - it wouldn't create an issue at all.
You don't know my siblings!

Obviously every family is different and in mine jealousies and hurt can come if one sibling is treated to something that another isn't. My other siblings all have older children so that's why taking the kids would not be an issue.
I haven't read the whole thread - but here are my thoughts.
Do you think your brother/sister (not sure which) would be personally offended by you making this offer? If not, then offer it. Either they say yes, or they say no.
If they say no, don't push it...it's their children, it's their decision. Yes, WDW is a wonderful place, but a child's life is not ruined by not seeing it.
I do not think that asking would offend them but I'm going to ask carefully all the same. If they say no I will not bring it up again.
Thanks again for all of the responses. I appreciate everyones thoughts & this discussion helps me look at the situation from another perspective.