If he was Camilla's biological father, why did he let Camilla's mother leave him with Cami...

most likely because the soul that was in camilla body was the wrong one, they explained in the beginning that being in the wrong body causes rejection and hatred even by family members

Wasn't Rivi is Camilla's older brother?

From spoiler in the earlier...
Their mother didn't leave Ravi's bio dad, but because Ravi's dad are dead already and for the Camilla's dad, when their mother have relationship with the dad, it's because the dad actually has lost memory at that time, and when the memory is back, he also lost his memories about them and then leaving Camilla's mom, Ravi and Camilla.

Ah, now that you mention it, did Camilla's mom mistreated her? I don't remember it that much at that point since she is a different soul, I'm expecting no one ever likes her when she's a kid.

I understand and I understand that, but it happened when Camilla was growing up. And it started to get worse when she was in the Duke Shorphel residence.When Camilla and OGFL met as children I thought Camilla was still occupying her own body.Or am I missing something? Because if it's like what you said, Camilla's biological mother should also hate her very much and won't take her anywhere. And only prioritize Ravi.

They are half-siblings, their mothers gave birth to them from two different men.

So that's how it is , alright I understand the situation....Now they were safe in Duke Shorphel's residence and that was all that mattered.

yea that’s the one thing i can’t fathom cause when she was in korea, her mother also defended her from her dad when she was a child so idk maybe you’re right, when they’re children it’s not that severe