"Some nutters who want to play a practical joke about magic," she says, because the Dursleys react with bizarre fervor to any mention of magic not explicitly disclaimed even when it ought to be redundant.

"Oh, that's ridiculous," he says, glancing at the letter dismissively—

—then he stares, his face going from red to green faster than a set of traffic lights, and doesn't stop there, going all the way to the greyish white of old porridge. "P-P-Petunia!" he gasps.

Dudley's attention is caught, and he tries to grab the envelope.

But Petunia grabs it first, then looks at it and raises a hand to her mouth in shock. She opens it and reads the first line, and stops there.

Okay yeah they definitely know what's going on.

"I want to read that letter!"

"Get out," she whispers, "both of you."

... 

She gets out. If this behavior on her aunt and uncle's part continues she still has the option to make a fuss later.

Dudley has a short-lived fight with his father and is promptly kicked out as well, and the door closes. He immediately puts his ear against the keyhole.

"What're they saying?" she whispers after a moment's conflictedness.

He glares at her and ignores her.

Tch. Should've known better. She stalks off.

Eventually Uncle Vernon returns to the living room to read his newspaper and calm down.

Dorea drafts and discards a variety of plots to get him to spill.

That evening, after returning home from work, Uncle Vernon knocks on Dorea's cupboard.

...Okay. She opens the door warily.

"...so, Dorea, about this cupboard. Your aunt and I have been thinking... you're really getting a bit big for it... we think it might be nice if you moved into Dudley's second bedroom."

Well. It's true, it won't be that much longer before she literally can't fit in it. 

"Alright."

"So—you can move your things there," he says, then turns around and goes off somewhere.

--Okay. 

She moves her things. There are a bit more of them than the Dursleys probably realize, sparkly rocks found on the schoolyard and cheap pretties bought with spare change they'd never notice missing. She is careful to keep them hidden among the ratty hand-me-downs as she brings everything upstairs. At least Dudley's second bedroom will have lots of hiding places.

From the other room comes the sound of Dudley bawling at his mother, "I don't want her in there... I need that room... make her get out..."

Ugh. She has no sympathy whatsoever.

He screams, whacks his father with his Smelting stick, is sick on purpose, kicks his mother, and throws his tortoise through the greenhouse roof, all to no avail; his parents don't budge.

Poor tortoise.

They don't bother Dorea, and are somewhat subdued the next morning. When mail arrives, Vernon tells Dudley to go get it, and he goes without complaining...

...only to cry, "There's three letters for her now! 'Ms. D. Potter, The Smallest Bedroom, 4 Privet Drive—"