1) We're not shown what "mother" fed her... Pretty obviously it was either a body or body parts: hands, arms, legs, heads, etc. (Note: On chapter 15 page 1, that the head from the image below obscures the floor between "mother" and Sephina. Body parts (or even a whole body) could be on that floor.)
2) That said... Sephina is supposed to be stupid(?) enough to not grasp that people don't like being eaten or seeing you eat other people? Little girl screaming in agony and terror as Sephina begins biting into and eating her, yet Sephina can't make the connection between "every time I eat people, that person and other people don't like it"? Even the most idiotic, sociopathic sociopath is not that stupid.
3) Others have speculated that "mother" is Leeseheimer: Seems quite possible to me. We already know that Tsuguto sees her as a monster.
4) (BTW, Is Leeseheimer an elf with a monstrous nature? Or something possessing an elf girl?)
5) What did the Dragon's Curse on Sephina do? Was it intentionally placed on her by someone? Leeseheimer?