Sequel?

ok, at the end of the movie (SPOILER COMING) Charles and his mother perish, so that little plot thread is at a dead end.

I disagree with this point because althought there is very little doubt in my mind that Mary died, Charles on the other hand is a different story. When examining the events of the ending his fate is more ambigious. From what we saw at the end, as Charles was in Sleepwalker form feeding on Tayna as Mary looks on and partoally transforms into her Sleepwalker form. Tayna blinds Charles by poking out his eyes. Mary attempts to stop her and says to Tayna:

"Stop, you're killing him."

Before Mary can pull Tanya off Charles she is attacked by some cats. Meanwhile, the Sheriff shoots open the door & incredously looks on as Mary fights with the cat. The sheriff shifts his attention to Tanya who is still screaming as she continues to gouge Charles' eyes out. The sheriff yells:

"Tanya get out of there!"

As Tanya gets up ready to flee, Charles grabbed her ankle very much alive causing Tayna to shriek in shock. The sheriff ran to her aid and as he approached her from behind she mistook him for Mary and began attacking him. Meanwhile more cats entered the house and attacked Mary. As Mary fought them off, Tayna stopped attacking the shheriff, got loose, and ran for the door. The sheriff briefly looked down at Charles who layed motionless. He then focused his attention on Mary who broke the neck of one of the cats. The sheriff took advantage of Mary's distraction and shot Mary killing the other cat in the process. Mary absorbed the shotgun blast relatively effortlessly with little side effect and fully transforms into Sleepwalker mode causing the sheriff toss away his shotgun and to flee in fright uttering the expelative:

"Holy Sh!t"

He runs toward Tanya and manages to open the door and they both get out. Tanya unsure about Charles' fate tries to confirm whether or not he's alive. The sheriff in fear for his life pulls Tanya away fleeing. He and Tayna have this screaming exchange:

Tanya: "Is he alive"

Sheriff: "No, no, c'mon we've gotta go..."

Sheriff pulled Tanya who resisted him wanting to confirm Charles' fate.

Sheriff: "C'mon god dammit."

Sheriff trips.

Tanya: "Sheriff, he was dead wasn't he?! He was dead wasn't he?!"

Sheriff: "Yes! He was dead! Yes! Let's go!"

Mary then invisibly exits the house and attacks the sheriff throwing him into a bear trap. Tanya runs to the police car and gets in and Mary follows attacking her. The sheriff throws a bear trap at Mary catching her in the head. Mary frees herself, grabs the sheriff, and impails him in a picket fence killing him. As Mary runs back to Tanya who was in the police car she was attacked by a multitude of cats causing her to combust in flames. A dying Mary screamed:

"You killed my son, my only son."

Tayna backed the car away as Mary wretched and shrieked in pain and she is frightened by and cuddles with Clovis watching as Mary burns. The reason I recap all this is because the last time we saw Charles he was after he grabbed Tayna's leg and she got loose when the sheriff saw him motionless. Personally I thought he was not conscious but still alive and the sheriff was in fact not sure whether he was dead or alive and just trying to get out of there. Same thing goes for Mary where in the heat of the moment she didn't bother to see if Charles was alive and went after the sheriff and Tanya. The most interesting fact is that Tanya who had attacked him was unsure the entire time as to whether or not he was alive.

As far as I'm concerned, they could do a sequel starting at that point with the Sleepwalkers that Mary had sensed earlier coming in, killing all the cats, and rescuing a badly injured Charles. But starting with this as a jumpoff point they could tie it to the past story through Charles while at the same time bringing a father/daughter sleepwalker duo like you suggested. Good idea by the way.