Mother cat having babies with son?

my girl cat is prego with her second batch of kittens the first litter she only had one live and so i kept it and its not evan a year yet and i have another outside cat witch im pretty sure is a boy. I let her out one night and i thought the outside cat got her prego agin....but now the kittens are havin a lil trouble one is i think blind in one eye or both but one is deffintly blind.....im just not sure if i should take them to the vet but i cant afford tat anyway so thought i would take them to the Human socity...how do i tell if they are inbreed or not?