Because all her people had become accustom of fucking their daughters. Many Fathers are above 50 years old. So to prevent such incident, canning hopes to deter such practise.
However, she does not realize that rules are for people to break. There are countries that legalized INCEST, take for example Spain, US, France, India, Japan, Korea, China .... even though it is legalized to have consensual sex among relatives, but these acts are quite minimum.
On the other hand, ME countries, including Mudslime countries have legal penalties and laws to criminalized INCEST, yet INCEST is rampage among their societies.
We can erect laws to prevent stealing, cheating, molest etc ... it is the societies that define whether law can be upheld.
Race and IQ