Tiff and Brenna can relate because Sterling went behind their backs and fucked their moms. They bring Amber into it because Sterling also fucked her mom behind her back. I assume their "revenge" will be on Sterling, or maybe their moms, or all of the above. Also I wonder what form this "revenge" will take, because from the teasers it seems to be Brenna/Tiff/Amber eating each other out?