It's weird but think of it this way- they were both kids when they were in the house together, even though they were bloodline aunt/nephew/niece they were being 'parented' by the same group of adults. Is it weirder than my two teenaged sweethearts becoming super siblings when they aged up? They married afterwards. Soulmates, super siblings.
The flavor text also mentions 'blood or not' I think?
I actually knew a uncle/niece who were in that situation IRL- they were in the house, he was the youngest of 12 and she was the first of his oldest brother's children. They were only a year apart in age- and the niece was the older, from what I can recall. They were both old enough to be my grandparents, but the children of the niece called the uncle their uncle, and their children called him Grandpa, and she explained that they were more like brother and sister than uncle/aunt because they were raised at the same time.
Gamewise, so long as the generational lines are more cut (the aunt never was under the age of YA while there were toddlers/children/teens of her sibling in the same household) then you won't have this happen. I think, technically, I think siblings (where one is a care dependent of another) can have the mother/father--son/daughter buffs too, but I'm not certain. If we can confirm that happens then we can see how much of this was a predetermined interaction, I think.
