This is just a random theory I came up with on the spot but what if Emilia is Satella's half-sister, it is probably incorrect but it seems plausible to me. Anyway, so the theory is that 400 years ago an elf and a human had Satella, obviously she went nuts and swallowed half the world but what if 300 years later (after Satella was sealed) the same elf had another child with another human (considering elves live for quite a long time this seems reasonable), so maybe the reason Satella and Emilia look similar is because they share the same mother/father (whatever one was an elf). I prefer this over the Satella is Emilia's mother theory but the theory stating that Emilia is Satella in the future just gone back in time would explain why they look exactly the same. Still I think my theory is interesting. (I have only read up to ch74 of arc 4)