When Joanna discovers of the incest between Cersei and Jaime, she does not inform Tywin. According to this link,
When Joanna learned of this, she send the maid away, moved Jaime's chamber to the other side of the castle, and put a guard outside of Cersei's chamber. Joanna then warned her twins that they must never do such a thing again, or else she would be forced to tell their father. Joanna died birthing Tyrion not long after this event.
I am particularly interested in knowing if Cersei, Jaime and Tyrion were born out of incest. Why am asking is because from the same link I provided above, I also see that Joanna had a brother named Stafford.
Why did not she inform Tywin of this illicit relationship?
Is it because, she felt the same mistakes she committed with her brother are now committed by Jaime and Cersei or due to the fear that Tywin might harm the children? Also, she dies when Jaime and Cersei were only 9. I do not understand how they could have engaged in some sexual activity at such an early age.