His father accused his mother of infidelity. That's why he treats him so badly. Not my kid. The Mother is to blame…
The father claimed Song Mo had an affair with a maid and killed her to silence her. I don't remember him saying anything about his wife though. He had a handin his wife's death as seen by changing her medicine prescription, her food, and all the old loyal servants in the manor.
That was probably opium she was smoking. They probably got high together all the time in their misspent youth.
It was a different era. Women were seen as lesser than men. The drama takes great pains to often point out that women can do just as well as men in many fields of life.
Fifth Uncle is really despicable. He'd ruin his niece's reputation and her future just to topple the Grand Secretariat and he's so shameless about it. If this was older Cdramas, FL would probably stab him XD