Technically they can both be sexually mature by six months, and yes, the answer to your question is the male will gladly mate with his sister if she comes on heat! However, if you are planning to get them both neutered at six months then I don't think you will have a problem. What could be a problem is if you let them out and a randy local tom cat who knows what its all about persuades your female to 'come up and see his etchings'. Your boy will probably have far more trouble persuading the local totty to put up with his immature advances.