Not that much younger. Lynette, the MC's Mom, is 18 when she meets Neil, and factoring in the time it takes to build a hotel, she could have been around 20 or 21 by the time she leaves her family to be with him. For Jill to be Lynette's sister, there would have to be a huge age gap, like if Lynette had the MC when she was 21, Jill would have to be 2 at the time making Lynette 19 years older, and nobody has vivid memories of things from when they were 2, so Jill couldn't possibly remember an older sister if that were the case.
The thing that debunks this theory the most is the picture in Jill's drawer under her bed that is clearly of her as a child of probably around 9 or 10 years old with what would appear to be her sister who looks to be in her late teens and who also looks nothing like Lynette.
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The "Jill Aunt" theory doesn't hold water and never has, not just because the game itself pretty much debunks any possibility of it with this picture and the age gap of the MC and Jill, but because the dev also debunks it and they're not going to go back on that.