Computer Generated Imagery. Meaning, there wouldn't be an actress there. Since Renesmee grows at a fast pace, they may need to just have a stand in then create a CGI figure over it. The wolves are CGI. They aren't actually there and, if Summit decides this with Nessie, then they wouldn't need an actress to play her.
I think she should spleet, split it at the moment Jacob imprints with Reneesme and then hears Bellas hart-, hart start beating... The 2nd part shoulf be ask she is waking and discovering everything.