My husband's parents wanted to take his daughter to Disneyland when she was two. My husband was a newly-divorced single dad and couldn't get off work to go. He was torn, he wanted to take her (he had never been to Disney World himself!) but he also wanted her to have the experience so he let her go.
To this day he regrets it. He believes that Disney is one of those things that it's the parent's job to take their child and see their faces light up at all the Magic. Now, granted she was two, but she still kind of remembers it.
We went not too long ago as a family. It turned out fine because it was so long ago that his daughter went, she went to Disney Land and we went to Disney World, and things have changed a lot since then. She got to experience Epcot, Animal Kingdom and Hollywood Studios. So it was like it was all new.
However, if you are planning on taking your daughter/niece later this year then it wouldn't be as magical with her going with her friend. So let the other girl take a different friend. It's not selfish.