Even though the eldest master of the Su Family is the head of the family, only a handful of people truly know that the one in real control is Su Yunling.
It's said that Su Yunling is the Young Master of the Su Family from the Ancient Martial Arts World, and the elders of the Su Family treat him with utmost respect.
The Imperial Capital Su Family is merely a collateral branch of the Su Family from the Ancient Martial Arts World, and it's by relying on them that they became the leader among the Four Great Families in the Imperial Capital.
Su Yunling is the Young Master of the Su Family from the Ancient Martial Arts World and has absolute control over the Imperial Capital Su Family, so to establish oneself in the Su Family, building a good relationship with Su Yunling is essential.
If Su Miaoyi truly gets together with Su Yunling, she can become the Matriarch in the true sense, and as her uncle, he would certainly benefit from it.