There would be way too many holes in her story to claim she is the shooter:
1. How would she even know about Edward's gun collection in his old office?
2. The security footage shows her arrive at home, which I imagine coincides with the time Drew was shot.
3. Why would Willow be crying like that if Nina was confessing--thus exonerating her?
4. Nina has never been to Liz's house.
5. Once the police ask her to walk them through exactly what she did--detail by detail--her story would fall apart. The killer would know that Drew was standing by the fireplace, when the first shot hit and then walked over to him to deliver the second shot. Forensics would disapprove her as the shooter with ease.
I think Nina is admitting that she lied and now the focus will rest solely on Willow as the suspect. But with these hack writers, who love to pen Nina as a whipping post, maybe she will confess to save her ungrateful bitch of a daughter.