I guess in about a month from now you?ll know whether you need to worry about disappointing your granddaughter or not. I think it?s important for the older sibling to see the new arrival in a positive light from as early as possible so we give each child a gift *from* the newborn at each subsequent birth. And to start even earlier, you could get the ?I?m going to be a?big sister? clothes for her from http://www.cafeshops.com/materni_tee/221696
If it is going to be a boy, you could look for story books appropriate to her age that are about a baby boy coming into a family. Either way, you may also want to read up on sibling rivalry in the book at http://tinyurl.com/34y9u
In the long run I?m sure the new addition will turn out well for everyone in your extended family. Best wishes.