I must say though, that sister has a sexy body, and I'm suprised the brother only got half-hard, despite the sexually suggestive state of her sister.
You'd be surprised on how (if their parents didn't step in and reprimend the offending party) that that's actually the norm. Most mammals are hard-programmed not to be turned on by their siblings to avoid incest and shrink the gene pool. We've been exposed to too much 'step' portrayal in eroticas that most people assume if you seen your siblings naked, you'd get turned on.
On the second panel Because the sister was distracted, she accidentally spilled the milk onto her naked body. The sister then apologized and asked if her brother got erect from the lewd moment (hence she said "Did you get up?", if I read that correctly). The brother then confirms that he did get erect, but only half-erect as he says in the parenthesis, resisting the lewdness of the situation.
The brother denies* getting erect, but the author note (in parantheses) mentions he was halfway there.
*Interrogative か used to indicate incredulity, usually translated along the lines of "Like hell/heck I did!"
