Forced to share first author title

For context, I'm a data analyst working in a public health/health services research lab. For the past 6 months I've worked on this project and did everything from-- data cleaning, analysis, brainstorming methods, to writing the manuscript. When the project began, my PI asked a mentee of hers, let's call him G, to supervise me. He met up with me bi-weekly for maybe 2 months, and was always confused about what the project was even about. He suggested some methodology that was then thrown out due to how bad it was. After that, he kinda dropped off from the project and just attended meetings here and there without contributing much. From my perspective, G hasn't done shit. PI came up with research question. As such, I very naturally assumed I would be first author.

When the time comes to actually start writing the manuscript, PI suddenly said-- maybe G will want to be first author. I was surprised and didn't say anything. Later on, PI unilaterally decided that G will be co-first author. When I asked her why G is co-first author, she said "G led the project before you joined," which is a straight up LIE, I know for a FACT that G was not involved in the project prior to my engagement. (PI has done this often-- lying when she wants to prove a point).

My question is: how normal is this in academia? I'm very early career, but every single PI I have worked with in the past have always given me credit when it's due (such as in this case, where I did everything). Is it fair for me to expect G to contribute significantly to the writing process in this case?

When I tried to ask PI whether G and I will be writing the manuscript together-- it seems like the expectations is still that I write it and G just edits. Is this fair? I can't tell. Do you guys have any advice on how to navigate this situation? From what I have read, I know that whoever's listed first will ultimately be perceived as having contributed most. Should I clarify that with my PI right now?