When Lady Chu married her father as his second wife, he was already ten years old and old enough to understand the ways of the world. Out of respect for the filial piety she showed the Old Madame, he was willing to address her as "Mother."
However, this was all based on the premise that they would keep to themselves.
Now that Lady Chu dared to have designs on Lan Qing, that was a different matter entirely.
Chapter 37