One of those girls is his sister, right...? That sure looked like the context, sister and her friend hanging out with bro in the room next door kinda thing......but I'm just guessing.
Oh, yes. One with short hair & white underwears is his sister, and the other with long hair & red underwears is her friend. Those girls are both virgin, so they now want to get lost their virgins as soon as possible with a little help from her brother...