In danger of inbreeding?

Question
Hi, I had a few questions about our cats. We recently adopted two littermates, male and female. We aren't sure of their exact age, but according to the vet's estimate they are about 4 months old now. We plan on getting both of them fixed when they are the right age, but I've read that some cats are capable of breeding anywhere from 4-18 months. Since they can't get fixed until 6 months of age, is it possible they may try to breed with each other before we are able to get them fixed? If so, is there anything we can do to prevent it? If they were to copulate, what potential dangers would there be for their kittens? We are pretty sure they have different fathers, since their mother was a SH calico, but the female is a LH with maine coon markings, and the male is a SH with a grey "tuxedo" and faint tabby stripes. I'm not sure if this helps in determining any risks, but figured I'd mention that just in case.
Thanks in advance!

Answer
Hi Jodi!
Your kittens are already old enough to be fixed. If I were you, I would go ahead and do it.  It used to be that Vets waited until they were 6 mos old, but now, as long as they weigh at least 2lbs, they can be fixed much earlier.  This way you wouldn't have to worry. There are way too many unwanted cats out there, no need to add to that.  Check with your Vet about getting them fixed now!  You'll be glad you did.

God Bless,
Becky