IMO the first thing to do is to find out whether or not your niece can see. Because only when you know, will you be able to help your sister to come to terms with her dd?s disability.
Living in denial will not help your sister, and it will not help her dd, but she may need time, and possibly even counselling, to come to terms with the fact that the baby she thought would be perfect, has something wrong with her. But of course in time, she will realize that her baby is actually perfect, just not in the way she first thought .
Obviously I don?t know the extent of your niece?s cp, and there are others on these boards who are better placed to be able to tell you how her cp might effect her life, but I can tell you with certainty that not being able to see will not mean the end of an independent life. I am totally blind and have been since birth. I attended school for blind and visually impaired students, both here and in South Africa, but these days it?s not uncommon for visually impaired students to go to mainstream school, as long as there is support to help them learn Braille etc. I am 32 now, I live with my dh and my 4 year old ds and I am a SAHM, but before having ds I held down a job as a finance manager in the civil service. I am totally independent, I can do everything that any sighted person can do, with the exception of being able to drive a car.
If your niece is blind, there is no reason why she will not develop in the same way as any other baby, go to school, achieve a-levels, and, if she wishes, go to university and have a career/family of her own.
Firstly though the most important thing to do is to find out the outcome of her appointment, I will keep everything crossed that the news is good, but if the news is not what you had all hoped for, then there are people on here who can help and support you all through it. And I am happy to talk to you or your sister about any concerns you may have.
Hth and good luck xx