Peviously: Jacob 'imprinted on Renesmee. Little did he know it was not real. The reason that it felt like a real imprint is the fact that he was really close to her mother and had a strong imagination. When Leah joined Jacob's pack she changed for the better. Jacob and Leah started to hang out more together. He started to develop feelings for her that he just could not egnor.
By that time Renesmee was full grown and Jacob realized his feelings for her has not changed. He stil liked her but as a sister nothing more. So he decided to marry Leah.
The Cullens and Jacob are still close even Leah does...