Nora, the answer is WELCOME!

I usually give them surnames, and the kid takes the dad's surname, but really the easiest way is to only breed them with people around their age, 'cause you can't have a mother and daughter who are both 18

The best way is to never tick parenting, breed them with people close to their age, and just add a last name(kids will/should take the fathers last name.. it helps reduce offspring/parental breeding, but is not 100% fix solution, unless you plan on pausing the game, every time your eyes are not upon your people.