"Years before, a recorded copy of Wonder Man's encephalographic traces had formed the basis of the Vision's artificial intelligence, making them something like brothers. Later, after Simon had returned from his apparent death, he had felt a strong attraction to Wanda - who was, at that point, married to the Vision. Now, with Simon returned from a second deathlike experience and having taken the Vision's place at the Scarlet Witch's side, there were still ties between the two."
Okay, stuff I missed while scanning through Wikipedia pages. I had figured the tension between Simon and the Vision was due to them being the current boyfriend and the ex-husband, but instead it's that plus the two of them are actually sort of related. Oh boy.