I think the first one die is grandmother next one is mother and final is the brother. Read back chap 26 then chap 27 you can see neighbor hear the mother scream but grandmother still on the bed with blanket on it (page 13) so maybe the first one death is grandmother next is mother and final is brother
I think mother is the one kill grandmother because police said find the boy with body full of wound, when the boy found out his mother kill grandmother, she gone mad and swing knife to the boy so that why he had wound, the boy take back the knife and kill his mother (i think is self-defense) and in this chapter we know that the boy feel anxiety and loneliness in his own house so i think when he accidently staff his mother he collapsed and when brother hear the noise he come in and saw his brother full of blood and mother die so maybe he scream make the boy scary and he kill his own brother
In chap 26 the boy said his mother and grandmother have a bad relationship and this chapter we can see that grandmother room is next to the kitchen where mother a housewife always work, this make the mother feel pressure when mother in law alway watching her + bad relationship between them make her collapsed too so she kill her mother in law
This house killing people from inside and you say that there are more than 100 houses like this ???? How many people will get collapsed by this kind of house ?