Hmmm... I think his goal is to instil inferiority I the twins, "this was the empress, noble regal beautiful and very loved by the emperor, he must also love the children with her a lot more than bastards from some random woman"
It's easier to manipulate them if they feel inferiority to the brothers and start seeking this kid for affirmation as their "friend", or at least that should be the plan, but the twins are already fully aware of their mother being a shitty person and even though the emperor remains a mystery to them, at least the brothers have shown honest, if confused, care.