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 Wölfe 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 teilt, split it at the moment Jacob imprints with Reneesme and then hears Bellas herz start beating... The 2nd part shoulf be ask she is waking and discovering everything.
