The two girls were not freinds and didn't know each other

Marie is in the car on the drive to the farm house with Alex.
Later on, she goes up to the guest room.
But I think she imagined all that up to the point where she goes outside for a smoke.
When she is outside, that is where her reality comes in. At that point, she is a killer stalking and watching Alex take a shower.
Then she goes back to her truck and masturbates fantasizing about Alex.
Notice that the killer doesn't ring the doorbell until she is done?
Now she is ready to go and kill the family and fulfill her desires for Alex.

I think being friends with Alex and meeting the family was all made up because how else could she have attacked the father at the front door like that and then reached in and unlocked the door?
It would also explain why she had the truck.
And when she goes up to the guest room it is immaculate as if no one had been there but only in her mind someone had been.
How else can it be explained the killer just happens to check the sink which she had just dried?

Damn I'm good.