Hi Muzzy!
It depends on what the breed is? Some of them ( females) could be capable to get pregnant at 5 months. Considering that you/ve got a very close related kittens - its better not to risk as kittens from such as mating could be brain damaged or something else....and you will be facing a very hard decision to PTS them. Most Vets do early neutering to stray cats. If your vet will refuse - tell him that there many others who will; plus - the boy kitten is easies to neuter at this age (at least one of them will be done.)

__________________
"Do not meddle in the affairs of cats, for they are subtle and will pee on your computer." --Bruce Graham
There are no rules, rules are only for people who fit inside a box, if you want to think out of the box there is no hope... Life has no Box. (Tony Howells)