Since Si Lingtian has already suffered losses, he naturally wants to ensure that after Shen Biran takes the child, she won't speak recklessly to Si Yuchen.
Right now, Si Lingtian has at most lost a tool to perfectly control Si Yuchen. However, if Si Yuchen discovers the secret that Si Lingtian has hidden for many years, it would push Si Yuchen further out of his control.
Shen Biran is true to her word. "I will continue to keep your secret, but I still want to advise you: Sometimes, keeping a secret from being known by others isn't always good. In case by fate, Si Yuchen discovers your secret, think about it — what's the difference between you telling him proactively and him finding out himself?"
If Si Lingtian were to tell Si Yuchen proactively, then while Si Yuchen would be shocked, he would gradually come to understand Si Lingtian, gradually understand his good intentions, and ultimately have a great possibility of forgiving him.
