Family relationships are sometimes a hodgepodge where it is difficult to distinguish the real bond between people.
In your case Arya, some would consider their father-in-law to be their real father.
I got this from a senior member Dr. Farr on https://www.astrologyweekly.com/forum/s ... hp?t=33837

"It is also my understanding that the 4th house is to be used -for father -(Alchabitius and others used the first triplicity day or night Lord of the 4th house for more specificity)
However, in horary, I follow Al-Biruni (and other early authors) and use the 7th house if the question is specifically about "another person", regardless of their relationship with the querent. So-in the tradition I follow in horary-the question, "Is my father trying to do X to me?" I would take the 7th as his (the father's) signifying house. Instead say I was asking, "Does X's father control her life?", then I would use the 7th for X but the key signifying house (quesited house) would be that of X's father, which would be the 4th house from the 7th.
But to re-iterate, in standard horary the 4th house is considered as signifier for the querent's father."

My grandparents lived on an island and everybody on the island was considered 'family'.
And one day my grandmother decided to play hide and seek with a neighbour behind the barn and she got pregnant. When the child was born, he looked like nobody else in the family... except the neighbour. Nobody ever said anything and he always was considered as their brother, a full family member. I wonder what answer would come out had he been subject to a horary question.

I'm updating to say that I got the results and they are not helpful. The ethnicity estimates are surprising and it could be either of them, and there isn't any close match that could give me more clue. Maybe I will get my mother tested so I can see what comes from her side, because the results are really confusing. Quite disappointed, but some of you mentioned that it could be difficult to get to the bottom of this, so you were right.

So I'm thinking of asking if some new information will come up in the future that will give me a definite answer and when would that be, do you think it's too soon to cast another horary on the same topic?