I guess I see a little bit more nuance. Jill wanting to keep the MC in the dark about her family is a perfect example of that, even if it is completely understandable. She also gets kind of evasive and even a little defensive at the MC questioning her about Isabella, even if his questions are inappropriate; nonetheless, she seems to get suspicious about the locked room after the MC asks her about it. There's also the question of the MC's foreshadowing (in his narration) after being stopped by Isabella in the library; though it's not a 100% certainty, there's at least a decent shot that Jill has some knowledge about whatever that plot point is.
It's kind of splitting hairs (and ultimately doesn't really matter) but I think Jill is a little less forthcoming on things directly involving the MC, so Sage's openness - at least with the MC, if not herself, like you said - is kind of refreshing to me compared to the other characters. Nonetheless, Jill is a contrast to the remaining girls in that she keeps to herself mostly, which keeps her intriguing in her own right.