Introducing the 'niece'

On the day of the barbecue...

"So! Walter, I've announced us!" Rebecca said as she sent a message via WhatsApp to the organizer of the grill festival.

"Then there's probably no backing out for me now," it came from what looked like Tina, Rebecca's just 18-year old niece.
Only, the voice was not that of Tina, but that of Rebecca's husband Walter!

"No, but just be careful what you say. You don't want to give yourself away too early or ruin my dear niece's reputation," Rebecca warned, her tone almost threatening.

"Your dear niece? Becca, let me tell you, she's got a thick head on her shoulders!" fake Tina retorted.

"What do you mean by 'thick'?" Rebecca asked, confused.

Now 'Tina' paused.

"Um ... " came her **** response.