How to deal with this, psycho 11 year old neice and supposed allegations?

It sounds like your neice has some emotional issues that she needs counseling for. To protect yourself the best option would be to have her live somewhere else. If this is not possible I would just keep doing what you've been doing. Ensure that you are never alone with her so that everyone knows her allegations are impossible. I understand why you are being nice but at the same time she is a child. Try not to allow her to blackmail you to get her way. I would also be up front with her parents and everyone else; again in an effort to protect yourself. At some point someone made her believe it was okay to make implications like this. I'm sorry your dealing with something like this and hope you are able to find a solution.