I think that's the toughest part of all of this. My daughter was 8
when she was diagnosed so we've been sort of slowly leaking
information to her. The best advice I heard was to deal with the
issue the same way you would deal with the birds and the bees:
carefully, honestly, and open to any question. One thing I would
stress is that blindness is not a certainty. Even someone with the
worst forms of Usher can expect to have 25 years of useful vision
minimum. With all the treatments that are under development, we will
have viable treatments within the next ten years. There is a very
good chance your sister will never fully lose her vision. That alone
might be a good place to start.
I don't envy you, though. It's a very tough discussion to have. Good
luck.