This is a genre of its own called alternate history.
When it comes to this genre, you have to do a lot of research to understand what effects the changes you make will have. The first thing you should research is the marriage between Louis and Marie Antoinette, arguments for it, arguments against it, how it was received, and just the whole marriage in general. Then you need to make your character fit nicely into history and you need a good reason for why Louis and your character would be married as opposed to other potential wives who actually existed.
You’ve got a lot of research to do if you’re going to pursue this particular moment in history, but you’ve also got a lot of options and possibilities for changing history.
One term that you should be aware of is Point of Divergence (PoD). This is the point in history where your story breaks off from real history. If you’re making up a character, the point of divergence is, at the latest, the birth of your character. If your character is younger than Marie Antoinette, then the PoD is whatever prevented her from existing.