Originally Posted by HollynCats
brother and sister can mate and they will, but there kittens will be deformed!!
A little dramatic, surely?! The kittens would not, necessarily, be deformed but, of course, there are many reasons why they should not be in a position to mate, not least the fact that they are brother and sister.
Females can come into heat as early as 4 months and males can become sexually active soon after, so I'd get them fixed as soon as your vet will do it. If your vet won't do it before 6 months you'll need to watch carefully for signs of your female going into heat and separate them promptly until you can get them done. I'm not sure what you mean when you say 'min 6 months' - do you mean it's illegal to get them done sooner?
It's perfectly safe to get kittens neutered earlier than that, and if you have a male and a female together I'd recommend getting them done as soon as you can.