Will cats breed with close relative?

I am feeding and observing semi-feral cats in my neighborhood. There is this mother and her one year old tomcat, who are very affectionate with each other. They play with each other all the time, and roll on the ground showing their belly. There are many cats around, including a daughter same litter as the tomcat. But they don't bond with the daughter/twin sister. The mother cat is also VERY aggressive towards other male cats, so her son is the only male in her territory. Is that normal? Should I interfere and send the son away, so he can get his own territory?