Aqua/Ruby and Himekawa Taiki have the same father. Genetics test fact. Their mothers are different (duh). Aqua made the incorrect assumption on which father they share and assumed that the twin's father was the man married to Taiki's mother. As Akane deduced, that's not the only and very obvious possibility.
Now do some more math and figure out what the next horrible conclusion that draws from that happens to be.