If everyone in a family is successful and responsible, that is extremely important and beautiful for the family's development, but for the Royal Family, it may be something they fear and dread.
The Royal Family needs useful ministers, but not a singularly powerful family that is widely praised, because they fear such a family might undermine the Royal Family's foundation and overthrow the entire imperial power.
Hence, some noble families, no matter how successful their descendants may be, won't have everyone enter officialdom to become a thorn in the eye. Even when father and son serve in the same court, someone has to yield at a certain age. It is impossible for both father and son to hold key positions, and a wise Emperor wouldn't allow it either.
To the twins, their father, Song Zhicheng, is the one willing to be mediocre for the family, and so is their fourth uncle.
