It works the same as the original puzzle. If you ask the lying small butt brother, he'll lie and say his brother would say he likes small butts. If you ask the truthful big butt brother, he'd say his brother would say he likes big butts, because he knows his brother likes small butts and would lie about it.
Essentially the negatives work out so that each brother answers with the kind of butt they themselves like, which you can then use to determine which is truthful (though at this point that somehow seems less important).