Yeah, agree with above. The sister being mad doesn’t really add much. If the mother was “mad” then that would seemingly add to the plot, as then we wouldn’t be sure what is happening is real or not. That also creates conflict in the movie as well as many would believe she is mad as she tries to warn others of the things coming through.

It seems to me that by channeling, it is like you are connecting to a person’s soul perhaps, especially if letting things through from the other side.

I would also add stakes by placing the boy in direct danger, that he becomes possessed by these spirits or something like that, that way we also get a tough choice for the protagonist at the end where she can close the portal and lose contact with her son, or leave it open but leave him open to attack as well.