Also according to the artists commentary. I can't fully read it so I won't try and translate it, but the first sentence I see says something like what are sister and brother doing, lol.
Taken completely literally, the initial artist commentary says, "Older brother and older sister are doing something..." The word for "doing" is in katakana, which is comparable to bold font or italics. From the perspective of the girl on top, it'd almost make more sense to alter the phrase to, "What are brother and sister DOING?". English doesn't have a very graceful way to pose this as an implied question while including the assertion that the two are up to something.
The secondary comment just says that there's another version of this image without the text, followed by "Goodnight".
