Just to be sure I don't misunderstand, the boys were teens, and at the same time you are saying the girl was 10, so I would assume/understand that the boys were probably at last around 13 or something like that, as in "start of (or maybe even fully, especially if older than what I indicate) 'hormonal age'" ?
From one side though, thinking well, the fact they did not have any special reaction may be less strange that one (myself included) could assume.
I remember when I was younger with a friend once we asked to a relative of his who was gynecologist (and married) if there wasn't some kind of frustration, excitement or something, days when he was visiting a number of patients, and his reply was that he was kind of "anestetised" in that context by the constant exposure.
Also, when I was around 13 I was definitively in very high level hormonal storms

(started at least a couple of years earlier), but I definitively would never have had any idea about my mom or my sister - from what you say, you girlfriend had probably been doing it already from some years before, so the fact they grew with it, and they were siblings may have been enough to neutralise even the teenager hormons.
In case someone thinks there is a contradiction with the fact I am making the incest patch for the game (by the way, if anybody is interested, yes, it's late, because I have some stuff in these days, it may come only next week-end), for me there isn't.
The incest patch is not neither about me (I am neither Myriam nor Marc), nor real life (as I like to repeat, a game should not be confused with real life).