I think your question is what are the signs that cats are mating? I am not sure exactly if that is what you are asking, but ok, here goes ...
The male will vocalize to the female and then grab her with his teeth at the back of her neck. He positions himself above her, sometimes using his back paws to kick her into the proper position and then does his business. When done, he will disengage, she will scream and then be very, very upset. She will roll around on the floor, she will hiss at him, she may even try to attack him if he doesn't get the heck out of her way.
Additionally, your girl does not need to be in heat to become pregnant by your boy. She doesn't even need to have gone through her first heat yet, as long as your boy is sexually mature and he can catch her, he will mate with her and the chances that she will become pregnant are very, very good. It doesn't matter that they have been raised like brother and sister. They don't think about it like we do. They do not associate their relationship as siblings - he is a male and she is a female, that's all.
The best thing you can do is have them both altered as soon as possible.