Trying to find this horror film

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Okay so –
I have a vivid memory of this film and watching it with my grandpa but haven’t been able to find it in literal years. It was a horror, and I’ll list the specifics I can recall.

There was a group of teenagers or young adults in a woodsy, swamp like area. I remember a cabin by a lake with a pier.
They start to be murdered, obviously, and in the end it’s revealed one of the girls is the killer, taking revenge for what I think was her dead sister or something.

There’s a scene where they pull back the covers on a bed and find a skeleton laying in it, as well as a fight scene towards the end where two girls scrap as the innocent tries to get onto a boat parked at the peer.

In the very end we’re given a scene of the girl giving the police a statement at the station. Once she walks out of the station she rips off a wig and reveals she’s actually the killer girl.

Any suggestions?

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@Clownkisses?

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Could be Fear Island?
Here is the skeleton scene in bed.
At 1:11:25

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