It's not even speculation, it's just people (and, by people, I mean idiots) deliberately ignoring the fact that the dev doesn't include incest in his games because he's not into it.
It is for this reason set in stone that they're not related.
I would never say anything is set in stone. Jill has a dead much older sister. Linette or whatever her name was, came from a very wealthy family out of town from the MC and Neil's current home. She died in childbirth. She was basically kicked out of the family at that time. There is the picture in Jill's bed drawers that appears to hold a blonde older sister. There are "Leaks" showing a page where Jill is introduced to the father, and he states that Jill is the mom's much younger sister.
I think it is safe to say Jill is not going to just put out to the manwhore MC that easily, so I could easily see that this could wreck the whole plan there by having that come out. Those are not items pointed out by idiots. Incest technically would not have been introduced. Now I have gone on record as stating that IMHO that would not be true to the published labels for the game and would break what was promised. I also stated that some kind of screwed up Acting Lessons moronic choice involving any of the girls (looking at Maya and Josy) and their survival, and that casting Bella in a negative role would be a huge mistake.
I personally think the non bombshell will be the relationship that pulls Bella and Jill to want to have each other and a single guy in their relationship would fit far better in the projected genre, and then your choice would be M&J or B&J. Granted an easy choice for me, as I am team Bella 100%, but it would not be easy for others.
Another bombshell would be knocking up any number of the HOTs and the other ladies. That could be epic with all his bareback action. That is the kind of stuff I could get into.
Back to your statement that only idiots think this, I think that was harsh. There is plenty to at least cause a pause and thinking on this.. The letter could be the announcement of a trust fund that was set up for the MC by his mom and her lawyers before she passed. Royce family money perhaps... Or maybe he is a bastard cousin of Rusty... I think it will involve the B&R royalty and it will put him in the place of taking Rusty's role in the DIKs which will fuck up things all the way around for the current power struggles around campus...
Wouldn't it also be great if Jade and Cathy are carrying his bundle of Joy, along with most of the HOTs????
The possibilities are endless, and yes they include Jill as his aunt.