If Lynette truly died giving birth, that would have been the right age for Jill to be 3. She is a third year student, thus 21 years old give or take. If she is slightly older or younger makes little difference. If that is true, one possibility we have now been given is that there are 6 diaries that Lynette had before the one that the MC just got. Jill could have read those and with her romantic brain lived out an entire life with her dead sister, and read about what her sister did with her before she died. Likely??? Who knows, but again, possible.
And just to be clear, DPC saying no incest does not preclude Jill as the aunt. We all Assume that we will get to close the deal with her. What if we don't get to? Then the no incest still holds.... just saying.