I agree with this, OR, call your vet and explain that you're keeping the entire litter and you don't want any accidents, and ask if they'd be willing to do either the boys or the girls earlier (usually the boys but who knows what your vet will say). Sometimes vets will tell you their preference, but will do it sooner if you have a good reason for it.Your other choice is to call around and see if another vet will neuter earlier. If the males are neutered the female kittens can’t get pregnant and can wait until 5 months.
Yikes, yes, a nursing mama and older kittens will eat you out of house and home! Things should settle down a bit now, though. . .until the kittens are about 8 months old, then you'll wonder how they can eat that much, lol.