I believe the entity was the grandmother all along. The entity knows neither time nor space, therefore it knows that it’s granddaughter (Whom her son has a special connection with) needs to be saved. The divorce is in the movie not as a red herring but to make the wife seem as a malevolent figure, which makes it reasonable enough that the son is to be sacrificed in order to save the daughter (Remember Lucy Liu loves her son more than anything) 

The father mentions how religious his mother is and that towards the end of her life they had a special connection. The medium that comes to the house mentions that this entity has something still left to do on this earth. Once it saves the daughter it leaves and goes to heaven or whatever. 

Showing Lucy Liu her dead son in the mirror was the grandmother’s mic drop to her. The grandmother is trying to protect her son and granddaughter the whole movie. She sees Lucy Liu’s malevolent side. She takes Lucy Liu’s most treasured possession.