this is the first time Mokou and Keine met, right?

or has Keine been devouring the history of Mokou's dead family all this time?

No, I think she just ate Mokou's existance.

Could we get a translation... I'm curious as to what she really did eat.

Probably the history of Mokou ever having a daughter...

According to the next part of the doujinshi, Keine ate the past self of 'this girl' and made her start a new life as Fujiwara no Mokou.