Alison's bad luck/bed
Looking back to the bed before leaving, she notices a weird lump of blankets, just beside where she was sleeping, it was just enough for a kid to be hidden, maybe one of her brothers thought it was a good idea to sleep with her and she didn't notice through the night.
Not thinking much about this, she pushes the blanket to uncover whoever was there and to her suprise, it is Jack, her 8 years old brother, much like her without his pants, and only wearing his long pajama shirt and a pair of briefs. Her eyes were drawn to his underwear, it had some sort of cartoon print, well at least he was sleeping, that way he couldn't tell where she was looking at.
Her face warms with the thought of seeing his penis, it is right there, only covered by his underwear. Her face flushes as she looks at him.