The thing is, as I (and, apparently, Kotone) understand it, those are not her memories.
She isn't the old Feng, she's a new person. It's more than just a lack of memories, Kotone says as much. Old Feng is dead, and new Feng isn't her.
Rather than "Feng resurrected", it would be more appropriate to say that Feng jr. is the daughter of Feng sr. and Kotone - sired through Old Feng's flesh and blood and Kotone's magic.
That being said, the secrecy around that is still an open wound, and Feng should really be told why Kotone used her magic to animate that corpse, why she calls her a "protector" but never lets her actually protect anything, and why she's clearly both caring and distant.
To Kotone, Feng is essentially a shared child from a dead loved one: Both a dear reminder and a painful memory. And Feng really deserves to know that.