Although Lu Mingzhen and his wife went to great lengths to cover it up, the Old Madam, having had children herself, knew very well what it looked like to have given birth, and she couldn't possibly be mistaken about a child's age.
Besides, Yao Suhe's favoritism was outrageous; she doted on Lu Shengqing to the extent that she wished to give him all the best things in the world, yet she was extremely harsh on Lu Lingche, often resorting to beating or scolding him, and sometimes even denying him food.
If Lu Lingche really were her biological child, even if she were biased toward her eldest, she wouldn't have treated the second child so cruelly.
Unable to bear seeing her grandson suffer, the Old Madam found an excuse to take the child into her own care and looked after him for a few years.