Are people purposing forgetting that Anisa was going to be killed by knights and had no memory of what happened after.
Its blandly obvious Dietrich saved her and knows Anisa is not his blood sister.
And what raised as siblings? Who is raising them?
Dietrich even said he did not even view Anisa as a child, family, or anything but a tool when we get his thoughts.
People making a problem for themselves for no reason, if you dislike him being the male lead then stop reading.