Hi, I have two kittens, brother and sister. They are 4 and a half months old and the male has started mounting his sister. I've phoned lots of vets and the youngest I can have them neutered is 5 and a half to 6 months. Are they capable of reproducing at this age or is it just 'practising'? The only advice the vet could give me is to keep them apart - not very practical.