Although Shangguan Yunqi seems quite formidable, she has too many weaknesses, especially her eldest son, who is in the prime of his career. Xia Bowen needs his heir at this stage.
The phrase "hesitate to act for fear of involving what one cherishes" wasn't created out of nothing.
After the plot was triggered, she had thought it through.
After Third Child Xia returns, he'll surely tell Xia Bowen about the loss of his little uncle.
But even if she presses down on Shangguan Yunqi, it won't help.
Because there's no evidence.
The little uncle is out of contact; either he's truly gone, or he's been controlled and isn't free.
Xia Bowen has no affection for his children from his former wife; even knowing his little uncle is missing, he'll probably remain indifferent.
Every possible path was blocked off, so she was no longer in a hurry.
If the little uncle is alive, it must be because he's of some use.
And would thus continue to live on.
