Even if everything you said was true Japan has a population 123,333,091 people according to worldometer, divide that by 40 and you get 3,083,327.275 people if we say a 3rd of those men are donating sperm even with rounding down that's 2 million people that could potentially father a baby, add to that the fact sperm can be frozen meaning women can get pregnant with the children of dead men, I think it would be unlikely. I'm not saying it wouldn't happen that people would meet their half siblings, I imagine it would actually happen a lot but the idea that everyone that is important in Sou's life is a half sister of either him or a blood relative of him is just too unlikely. That being said what's likely and what's canon are 2 different things, I just think the "it's too imbalanced everyone must be siblings logic" doesn't take into account just how many people there are.