I didn't quite understand this one, it was pretty confusing, Imma just assume she was a ghost girl trapped in the doll.
No, like Pekoe put under spoiler it's actually her twin sister who die before birth.In thoe kind of situation it can happen that the dead foetus is absorbed by the other one. That's what the mom mean by "Onee being a part of you", Mom was most likely gonna be birth to twins but the Onee die and get absotbed by Mii. The doll most likely represent the dead sister and Mii throw in the fire can mean she understand the situation and get over it
I don't know about that, just a minor detail: If Mia got absorbed? How did they know it was twins? That sort of thing happens very early, I think. I'd guess Mia was stillborn because the two of them were very weak (Mom wasn't too healthy maybe?) so Mom says that about her lost sister. The doll was a surrogate for the lost twin?
Anyhow, quite a plot-packed story! Do like! Even if it's kind of sad.