It's kind of tragic he could never take that command/advice to heart. More than a decade after this flashback's time, he's still trying to tell people what he imagines they must want to hear, in order to blend in.
@dkuro Little by little. It's not so easy to change one's very personality. Being already an adult with his own life, his interaction with the old family would also naturally decrease, which would both help with it but also make any change less necessary.
The manga has a strong focus of the main guy trying to accept his family, it would be really disappointing if they ended it with the main girl hooking up with her non blood-related brother.