He watched, stroking his erection, as his sister got ready for her bath.
Unaware of her audience, she also practiced a little "self stimulation".
He assumed she knew he was there, and that her "show" was for his benefit.
But when he came out of the shadows to give her what he thought she wanted,

she RESISTED . . . and then he INSISTED!

You know what they say about "ASSUMING"!