How big was the age gap? I've noticed that unrelated sims, (if they were in the same household before either turned into YA) can get sibling buffs. Moved my teen GF into my teens household. Upon being YAs....they're now "super siblings".
It may be a factor. The aunt was a teen living in the same household with her sister, nephew and nieces.
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?
