We have a new kitten...a stray who just wouldn't go away and made it into our hearts when she was out on the front porch with a skunk one night.
We took her to the vet yesterday and she is 4 months old and the vet said they spay cats at 5 months. So we go back in four weeks for that. I believe males can be neutered at the same age.
Is there a no kill shelter in your area that could possibly take the kittens. You may want to check before spending a lot on them. But if you're going to keep them definitely get them fixed. If they are brother and sister you could end up with some really messed up kittens to deal with.