Toothbrush with suction cup.
Big sis is a virgin Futa and not sexually hyper active. Only masturbates for a normal active 19 year old, but knows she's bi.
Little sis is a 8 year old girl. Not knowing the word sexy or any innuendos.

Cheesy ad. Only to sell kids a useless gimmick.

Cub tries out many surfaces and shows friend. All innocent and none suggestive so far.
Big sis comes by closed bathroom door, hearing "Look, I can brush my teeth hands-free!"
She smiles and expects nothing bad.
She sees her sister on her knees, bobing her head back and forth on the toothbrush. White foam dripping. Big sis films this, not sure what to feel. Little sis thinks nothing of it.

Big sis reading magazine. Cub says she can't find her tooth brush.
Big sis nervously says "Errr... I don't know where you left it." Big sis remembers how she rubbed the toothbrush between her labia, while watching the footage of little sis felating the brush. 
Big sis hiding her face behind magazine. "Maybe you left it at camp?" Cub is sad.

Big sis sees ad for electric brush with suction cup and unique design. Nervously stares at it.
Bis sis gives it as present to little sis.

-Later that night.-
Big sis uses the electric tooth brush to masturbate and watching new footage of cub blowing the brush with suggestive foam and drooling from her mouth.