How do you deal with a difficult niece?

My question would be, how old is she and in what way is she difficult? I'd say that kids these days are a lot smarter and intuitive than we think, so the answer to your question may be getting to the root cause of whatever it is and then addressing it from there :)

If she's old enough, you could try to talk to her and find out why things are tough between you two. But otherwise, you could always speak to her mum.