Hi Sarah,
We have 7 cats that all come from the same family, (we have mums and sons and daughters, although non of them are in bred as they boys we have were nutered before anything could go on!)
We have lots of play fights going on, and some times between Smokie & Sammy some real scraps kick off, but no harm is every done.
We have always wondered, if we were to bring any of the kittens we rehomed into our house, would our cats realise they were related!?
So I suppose that hasnt really answered you question in anyway, but just thought I would have my say :-)