So your sisters and son have PKU? Well if your son has PKU that would mean that you and your partner would have to be what are called carriers (if your sisters have PKU then your parents would be this aswell). It just means that you both have a PKU gene, but you wouldn’t have PKU because you have to have 2 PKU genes to actually have PKU, which would be what your sisters and son would have. Hope this helps you understand alittle bit more! If you have any questions at all feel free to message me here.