He talks, has lust, is literate, and has a general grasp of right and wrong. Mentally (and physically if i had to guess) he's old enough that you want to begin discouraging the behavior. Especially since bad habits apparently can turn him into an oversized monster.
Spike is also a child, you know (or at least is supposed to be. I don`t know what the artist meant his age to be in these comics) and I find jealousity really understandable reaction here. She`s proud of her mature beauty and now a male person who`ve been admiring it is admiring (in front of her, no less!) some less mature looking girl instead. Although, for her to be jealous over something like this, she would have to respect Spike and his taste (when it comes to beauty), which I actually find quite beliveable.
Not to mention she might also be angry for him to look at her little sister with such a lustful eyes! Although judging by the "Hmph!" at the end, I think she`s rather personallity hurt or something.