Your question isn't very clear.
If your niece has PR status then she can work in Canada.
I don't understand what you mean by "she has done her medical from Russian university". If you mean that she has completed her medical degree in Russia and wants to know if she will be able to work as a doctor in Canada when she arrives here - then the answer is no - she won't be able to work as a doctor in Canada immediately. She will need to go through the process of being certified as a doctor in Canada first. This process typically takes several years.
