Why do brothers and sisters in Jin Yong's novels fall in love, and there is basically no happy ending?

If brother and sister fall in love directly, there is nothing to extend. Jin Yong's male master has never written. It can be said that his sister was the first woman he came into contact with. In order to expand the emotional line of the man, it is necessary to break the original sibling relationship. Once it is broken, the female host will have a chance, and the good ending will naturally be left to the male and female hosts.