Why don’t the twins notice anything?
I mean, come on:
- First year: "Hey, Voldy's balancing on Quirrel's head."
- Second year: "Hey, our sister is going somewhere that doesn’t exist."
- Third year: "Hey, some dead guy and a mass murderer are under the whomping willow, along with our teacher Snape, Harry, Hermione and our brother."
- Fourth year: "Looks like Voldy's in a maze, with the champions."
- Fifth year: "Hey, Harry’s on the train, it looks like he’s stunned."
There are way more. This is the tiniest list. And the same thing with how Lupin could see his friends.