My speculation of Seiichi’s mother being a psychopath were true, as i had said in my previous post.
Now chapter 20 shows us that the mother is not mentally stable and she has an emotional outburst and all.
Now as for her son (Sei), she feels that he was always left alone (but i don’t think thats true). She is less of yandere but more of a dotting parent (to an extreme level), well the manga showed a little possibility of incest but if that happens then she might be a yandere.
There would be some major events taking place in the upcoming chapters.