A novel about a cousin who visits the protagonist and is very suspicious

The cousin is a goth girl who is very suspicious and the protagonist grows weary of her as she is visiting her after I think recovering from a traumatic car accident? Later on the protagonist’s dog is killed and towards the end of the novel it’s revealed that the cousin actually stole the identity of the protagonist’s cousin and that her real cousin was either murdered or actually died in the car accident I don’t really remember. I read this for a middle school book report and looking back the ending kind of gives me the visit vibes with the plot twist. I think this was also a horror or thriller novel too.