"Definitely"?

Listen to the audio commentary. The ending of the film when Alice is walking away. The OP could have a point. I may not agree, but just listen to the audio commentary.

That doesn't negate the fact that the person who killed Karen was obviously an adult.
There are 8.2 billion people in the world. 8.19 billion of them have never heard of and don't give a fuck about Charlie Kirk. Get over it.

Pay close attention you say. KAREN was stuffed in the box. KAREN was strangled. ALICE was the nutty sister. Pay close attention.

possible spoilers
If you pay close attention in the beginning, the masked killer who strangles Alice is a lot larger than her, its just hard to notice it because of the camera angles. Take a close look when the killer is stuffing Alice inside that wooden chest and even kneeling down you can tell the killer is a grown person.

You didn't pay attention to the film at all if you believe that

It's true there are other possibilities, but William of Occam would be spinning in his grave over most of them.
There are 8.2 billion people in the world. 8.19 billion of them have never heard of and don't give a fuck about Charlie Kirk. Get over it.

I agree with you. I just kept getting this gut feeling about it
.

Or..
Alice did kill Karen. Tredoni then went on her own killing spree against the sinner parents. She attacked Annie, thinking it was the mother. When Tredoni ran out of the complex, she snuck out through the basement, and encountered Alice. Alice believed it was Karen, back from the dead, looking for vengeance. Alice would've had Karen's cross necklace in her collection, and then Tredoni probably took it at this moment.
The other clue that Alice killed her sister, while Tredoni did everything else, is in the MO. Karen was strangled and then set on fire, while the other victims were all stabbed by a knife that was carried around in a bag.
I just find it funny how so many people on this board are quick to say "Tredoni did it" without considering any other possibilities.
Welcome to
Fright Night
.for real.

Yup,
The old lady killed Karen because the Father gave her the cross instead of her. Also, her daughter died the day of her first communion years ago.

Dont any of you pay attention when watching movies??
The old hag had the cross that was stolen from Karen when she was killed, then it ended up in the dad's mouth when she killed him.
BOOM BRAAAP!!!
"What your mother and I must know, is.."

Absolutely no way. Just one killer, not two.

I kind of suspect two/dual killers. Alice killed her sister and I also suspect stabbed the aunt. Of course the old lady killed the father and the priest.

During Aunt Annie's stabbing, Alice was
in the basement and she must have seen the yellow-coated figure and then hid, thinking this was Karen.
She quite possibly believed what Alphonso had previously warned her about happening - Karen not resting easy and coming back from the dead.
I don't know why Alice was thinking that yellow-coated figure was Karen. Karen was tiny compared to the old lady.

No, absolutely incorrect.
SPOILERS BELOW
During Aunt Annie's stabbing, Alice was in the basement and she must have seen the yellow-coated figure and then hid, thinking this was Karen. She quite possibly believed what Alphonso had previously warned her about happening - Karen not resting easy and coming back from the dead.