My previous supervisor keeps asking me to review her papers. How to deal with this? I am currently a post-doctoral researcher at some institute. My Ph.D. supervisor, from my former institute, keeps asking me to review papers that come to her for review. I am pretty sure that this is because I am good at a particular topic which is not her expertise. She has coauthored papers on the topic but she is not an expert there. 

I have two problems with this. First, since I am not her student now, she should stop doing this; I am an independent researcher now. Second, I feel that this takes away credit from me. She could tell the journal that I am a better person to be a reviewer and this will benefit me since I am still young. Currently, I use my skills, time and energy to review papers for which she earns credits and I lose a chance for the same. Should I tell her this or is it normal in academics? If I should, how should I?
No it should not be normal. It is the decision of the editor to pick a specific reviewer. As a reviewer, you are supposed to keep the manuscript confidential. While i would see that this can be argued if you ask somebody working in your organization and reporting/being supervised by you, i think giving material to somebody whom you do not have direct control over in another institution is a pretty bad violation of the usual journal policies.

There are a number of bad situation which could arise from this 


You actually having a professional relation to the peroson which you review (without your former professor knowing) 
You telling your new boss about this, and he/she deciding to do politics using that knowledge (delaying etc)
You plagiarizing the paper, without the editor being able to track how that ended up there (since nobody knows about it)
You asking for changes to include references to your papers (innocent, since the editor team wont catch it...)


So no, beyond you not getting credit, i think it is a pretty shady thing.

(Added after comment): There are essentially two options, which depend on the mindset of your supervisor. 


You explain what I wrote and tell that this makes you uncomfortable - you don't have to tell about her taking credit for your work, so it is less personal.
You inform the editor about the incident, pass your review, and ask the editor to be confidential about it. (I would imagine that they may just remove you former supervisor from their list)