Not again. Here we go.
Factoring in the following...
- Jill is 2 years older than the MC.
- Lynette wasn't pregnant when she ran off to be with Neil.
- Pregnancy generally takes 9.5 months.
- Jill said "I would read, she would write, we would talk and listen to music together."
It would mean if Jill were MC's aunt, she couldn't be more than 1.5 years old when Lynette ran away, and most likely less than 1 year old. And Jill would need to be a genius toddler to be able to do all that with her sister.
Couple that with the fact that DPC has been very clear that there are "no incest" in his games.
So no, Jill is not MC's aunt.