Can I claim my fiance's daughter as a dependent if we split everything for her 50/50? I'm not sure if she already claimed her, she might have, so also, can we both claim?

The daughter of your fiancé can only be claimed as a dependent on one tax return.  If you are not the father of the child, the mother of the child has first claim for the daughter as a dependent.  Also, if the child has not lived in your household for the entire year you would not be able to claim her.