Do I have any legal ground to stand on?

Legal grounds for what?  What do you want to accomplish? If your niece borrowed photos and now is refusing to return them, she may have breached your agreement and committed the tort of conversion.  You can sue her to compel her to return the photos and, if she can no longer return them for whatever reason, for damages for their loss (although I don't know what value, other than sentimental, they may have had).  Also, she may also have committed a criminal act, although I doubt the police will want to be bothered with this family dispute. If she is telling people that you have Alzheimers while knowing that you do not, that could be considered defamatory.  You could sue her for libel and/or slander (depending on whether the defamation was oral or written).  You may be able to obtain an injunction against her repeating her lie.  You probably have not suffered any actual monetary damages from the defamation, but there are some knowingly false statements which are considered defamation per se, where the defamed person can recover money without needing to prove any actual monetary damages, and a false statement that one has Alzheimer's may qualify.