Sister in law wants to be the godmother... I’m not religious.

I don’t come from a very religious family. My moms side is catholic and my boyfriends family is too, but my mom chose not to have me baptized so that I could decide what I wanted to believe in as I grew up. I don’t really have a label for my beliefs. Anyway, my boyfriends sister isn’t a god mother to any of her nieces and nephews so she wants to be the godmother if we baptize our daughter. The only thing is that I’m not planning on baptizing the baby. Not that I am necessarily against it, it just wasn’t something that held high priority with us. What exactly does being a god mother or god father mean in catholic religion?