Question about authorship

A fellow grad student in my lab initiated the drafting of a review paper because her committee recommended that she should. Not sure why. Me and another student are helping. I am writing the introduction and background, as well as the conclusion, and editing the whole thing, including integrating the sections. She is the first author and I am the second author. I feel like my contributions are kind of substantial. Would it be appropriate to ask to be a co-first author? How does one determine who deserves first vs second author, or shared first authorship?