When Edward injects his venom into Bella he didn't know that there was someone else inside her. Callesme Edella Cullen was born 20 minutes after her twin sister by her own. She was a brighter colour than Renesmee in all aspects. She was more vampire than Renesmee resulting in her growing much faster than her sister.
Renesmee envies her seemingly elder twin sister and this sibling rivalry goes in one way. No matter what, Callesme loves Renesmee with all her heart and no one, not even Renesmee herself, can change that. But things get complicated when the Volturi comes to visit.
Renesmee isn't the only Cullen imprintee, Callesme is too. Paul Lahote realizes this too little too late when Cauis Volturi takes an interest on the same girl.
(I had a copy on Quotev) (The more people love this, the more I update)