First of all, we'd have to know what the meaning of siscon or sister complex is. According to Wikipedia, Sister complex (Japanese: シスターコンプレックス) is a state of strong attachment and obsession to sisters. It is commonly abbreviated as "siscon" (Japanese: シスコン), and in this case, it is also used for brothers and sisters who have a strong attachment and obsession to their sisters.
And now, let's look at how Aqua acts. He does kind of act like he's very strongly attached to his sister. But there's also another factor why he acts like that and it was the death of his mother, Ai. And also, by the way, Ruby is quite literally the only family Aqua has left. And with the presence of these two factors, it makes sense why he's strongly attached to his sister. He's attached to the point of crushing her dreams because he didn't want her ending up like Ai.
But there's another part to being a siscon, being obsessed. Is Aqua obsessed with Ruby? I don't think so, I think he's just obsessed with protecting her. I mean, as I've said, she's literally his only family left. He isn't obsessed with her really, he isn't fixated or anything like that at all.
But there are also other meanings of siscon that I've found but it was on Urban Dictionary.
I don't really think Aqua is this, when we're in more romantic panels between Aqua and a character. Those characters would either be Akane or Kana. I think he never once saw Ruby to be anything like that. Aqua probably just sees Ruby as his sister or anything familial.
So yeah, my conclusion is, Aqua is not a siscon. He's just some guy who wants to protect the only family he has left.
