Jinx is the middle kid of nine- he has four younger sisters and four older. I haven't bothered to work out ages or names or anything (I doubt I'll ever have any of them appear in the strip, other than maybe in passing) but if we assume roughly human terms (IE, his mother didn't have two "litters") and we further assume that Jinx himself is about 11 (which is how I write him) then the oldest has to be at the very least 16 or so, more likely 18, and the youngest no older than 7, probably more likely 5.
So, we can further assume that at this point probably only the oldest 2 or 3 are... "developing", I suppose, for want of a better phrase.
Besides, they're sisters, and this ain't Arkansas. I can easily picture Jinx kind of "tuning out" the sisters to the point where accidentally seeing one of the older ones semi-uncovered is nothing unusual or even noteworthy, whereas if he saw Sandy in the buff for some reason, that might very well be a "eureka!" moment for him.
However, I have no plans in scripting anything like that. Jinx is a far better character as his semi-naive self, and the last thing this strip needs is another horn dog.