Parents always want to give their children what they didn't have, give them options they were never offered. Rose wants children, but can't have them. In this scene she is acting as a sort of mother for Bella by making sure she has a voice for the option that she didn't have.
Twilight: The earlier movies within the saga depict Bella, the female character, as weak and in need of make intervention. Fortunately, once Bella gains supernatural powers similar to all those around her, she is seen as being just as strong and confident as all the other characters. In Breaking Dawn part 2, she is a strong and brave person who will do nearly anything to protect those who are close to her.