Imperfect parenting. Forget the superhero stuff and go back to what made the FF relatable in the first place. Superhumans had very human flaws. Have Sue make some common parental blunders like comparing her children to other people's children. This would require the FF to have a supporting cast with actual children, but that's a different discussion. Have her embarrass her kids in front of their friends by reminding them to change their underwear like Delia Ketchum would in Pokemon. Maybe have Sue work a lot and not see her children as often. Have her punish her kids too harshly. Do something to make her less of a generic mom figure and more of a flawed but well meaning parent.