Will brothers and sisters mate?

Now that I have 9 kittens, I was wondering if the brothers and sisters will mate with one another? I have a feeling that they will given my experience with other animals.

What is the age that they would start doing this? I am hoping to give them away at 8 weeks, is there ANY chance that they could get pregnant before then?

Yes, they definitely will mate with each other. I've heard of females getting pregnant as young as 4 or 5 months. The kittens should stay with their mom until at least 10-12 weeks old, but they cannot get pregnant at that age that I am aware of.

4 months is usually the earliest, but it could be possible, earlier, but probably not sooner than 3 months.. Kittens really need to stay with thier Mom to learn many things from her until at least 12 weeks old, to give them the chance to develop properly. Is there any way you can keep them until 12 weeks?

and yes brothers and sisters will mate.

Humans are pretty much the only animal where brothers and sister's wont mate and even with humans that isn't always the case. Also in your situation I would give the kittens away to good homes at 8-9 weeks just because there are so many of them and it's probably terribly hard on the mother to care for them all.

Yes siblings can and do mate. Its better to keep everyone together for 10-12 weeks before letting them go. 8 weeks is still too young. I would say the earliest to worry about getting pregnant is 4 months.

If you are keeping any of them, get them spayed/neutered by 3 months old. It would be ideal if you can get them all done before they are placed.