For the case to be true, their mother would have had to have had sex with and be impregnated by her father which would make the child she bore her brother (because he is the son of her father) & son. Then, if she had sex with and was impregnated by this son (that was her brother and son), the child born would have a father that was his uncle (mother's brother), father (obviously, since he was the one who impregnated their mother) and brother (because he is also their mother's son, just like they are). Had to think about that one for a bit to figure it out LOL
