WAIT SQUIGGLES!!!! I don't think there is a need to eliminate him just yet.
Not only was this incident an accident, but it appears that this event is bringing about a shift in the attitude Nobara is taking toward their relationship.
While he does seem to be fairly infatuated with Yukino by how much he was embarrassed when he caught her. (I mean come on who wouldn't be with how cute she is)
But he did not actively make such a situation happen, so I believe this can be forgiven should he not actively commit future sins.
Putting a warning shot over the bow or at his feet and keeping a close eye on him is enough for this infraction.
Nobara's Brother already gave a warning so the only thing left to do is keep him under our watchful eyes.