Flower Girl/Gender Role Problem - Advice Requested!

I do not want a flower girl at my wedding. We're trying to minimize gender-specific roles as much as possible (one example is there will be no difference between bridesmaids and groomsmen) and to that end, I would feel very uncomfortable with a flower girl. My mom keeps bringing it up that my brother's future wife's child should be our flower girl so that they feel included. I have a strained relationship with them to begin with and didn't even want to invite the child to the wedding, let alone give her a role. What's a polite way to tell my mother that this isn't on the table, will never be on the table, and I'd appreciate for her to stop bringing it up? She keeps pulling FH aside and telling him to "work on me". I don't want to give her false hope just to make her more angry when it doesn't happen. If it matters, FH and I are more or less splitting the cost of the wedding with my parents.