Without any context, it might seem like Rosalie is talking about indoctrinating Bella into a creepy cult when she says, "Go blather to someone else about the joys of becoming a newborn." Rosalie and Bella certainly don't get off to a great start as future sisters-in-law. However, that all hinges around the fact that Rosalie doesn't want to take humanity away from someone who has the chance to live it — unlike the Cullen children Carlisle turned when they were dying. While the quote sounds like she's talking about a religious cult, in reality, she just doesn't want Bella to give up all of the things that life took away from Rosalie.
The line itself is pretty ridiculous and undercuts the severity of the moment, but in reality, Rosalie is the only Cullen to truly consider Bella's future. Many teenagers makes rash, bad choices when it comes to young love — and they usually live on to regret them when they've gained some age, perspective, and a few wrinkles.
But by Bella making a life-altering choice at such a young age, there's no saying what human milestones she'll feel cheated from when she fails to age. Given Bella's future stint as a fiercely protective teen mom, she would feel something's missing if she never had Renesmee — a scenario no one thought was possible. Bella clearly yearned for motherhood (and other human moments) without even realizing it — because what 18-year-old would? But Rosalie wanted Bella to see that there's more to life than being a vampire.
