In his most difficult days, having a little companion by his side must have meant a great deal to him, right? Perhaps that young girl played with him, gradually cheering him up, or cried with him, so he wasn't alone anymore, or laughed with him and made him feel less isolated... Growing up as childhood friends, even if it wasn't love, it would still be a moving kinship.
Xiaoqi had a fondness for the woman named Fangcao, and his love, likely borne of time spent together, probably had a mix of familial affection as well. Such feelings are the most resilient; how could a few obstructive words from his mother make him give up easily?
Unlike Old Lady Cui, Cui Qingrou often visited Cui Pingan at the estate during her separation. That was when she had encountered Fangcao on many occasions, hence understanding her temperament and character quite well. She found her to be a simple and kind-hearted girl, which is why she wasn't so opposed to her growing close to her nephew.