If you don't let them go because you want to experience the joy of watching them at WDW yourself, but know in the back of your head it's not going to be financially possible, that would be just plain selfish
THIS IS WHAT THE OP SAID:
"It is a dream of
MINE to visit wdw with my two nieces."
"
It would be their 1st time to the world and by the time the trip is here they will be 9 and 7 years old.
My brother & sil want to take them someday but
I fear that it will never happen & the older of my nieces is very precocious
so I'm afraid the magic will be gone for her before too long...the age of not-believing and all that."
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.
I am a middle-aged man. I went to DW for the very first time two years ago. It has changed my life in ways only people who frequent these boards can know. Only the people who have been to DW, and felt the magic, and seen the kids, and hugged Mickey, and ridden TT, and visited the HM can know what I am saying. If I can wait four and a half decades and STILL feel the Magic, those children are not in danger of missing out. They are simply not getting it soon enough for the OP.
There is absolutely no chance those kids will miss the magic, whenever they do go with their parents who obviously WANT to take their kids.
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.