As a female, it took some time for me to realise that it is not unusual for men to feel some physical attraction when they look at women in their immediate family (even a daughter or sister), especially if they are significantly uncovered. Also, that many men do subconsciously "track" the attractiveness of female relatives (even if they themselves would never act on it and would deny it, but still they may be subconsciously evaluating how attractive they may come across to other men). 

Of course given the taboo of incest these thoughts are usually shoved aside. However, sometimes they come out, for instance, some men become uncomfortable when their daughters pass puberty and develop a womanly figure. Or sometimes in sharp comments about clothing and behaviour, etc. 

I say "as a female" because I don't think most women tend to feel the same way about male relatives, and the majority of women would probably respond with something along the lines of "Ewwwww".

[An exception would be with respect to blood relatives who never met in childhood, for instance, a half brother and half sister who meet each other only in adulthood. There is some psychological phenomenon that happens whereby it is not uncommon to feel some sort of attraction, in which case that could also be taken into consideration here, even though technically they are mahram and certainly a marriage is unlawful.]

I am not saying that this always happens, but just that it isn't an unusual dynamic and is one that it took me time to put into words. So it could be considered, at the end of this day, this comment either will apply or won't apply to a person's family dynamic. 

(Also I apologise for any generalisations in the above, generalistions happen but I do acknowledge the perils of them!)