In episode 13 of the anime,
it's revealed that Kazuya and Kanon aren't biological siblings; their parents (Kazuya's mother and Kanon's father) married each other after both children were already born and old enough to walk and talk. It's heavily implied that Kanon's mother died due to some illness
but nothing was said about Kazuya's father.
Does the original game (or otherwise another official adaptation of the game) explain what happened to Kazuya's father?
According to various Japanese sources,
Kazuya's parents divorced.
From Aniota Wiki -spoiler part- (Japanese),
実は果音とは血の繋がらない義理の兄妹である。一也の母(離婚)と果音の父親(死別)が再婚したためであり、再婚したのは10年以上前である。前田姓はもともと果音達の姓であり、一也達は再婚してから前田姓を名乗るようになった。果音が義理の妹であることを一也は知っているが、果音には隠している。しかし、わだかまりがあるわけではなく本当の妹のように大切に思っている。また、兄として果音を守ってあげたいとの感情から果音とはすぐに打ち解けいてるが、果音の父親と打ち解けるのは少し時間がかかったようである。 (Emphasis mine)
Rough translation:
Actually, (Kazuya is) Kanon's stepbrother. Kazuya's mother (divorced) and Kanon's father (bereavement) remarried more than 10 years ago. Maeda is Kanon's surname, and Kazuya's family took it as their surname after their remarriage. Kazuya knows that Kanon is his stepsister, but he hides it from her. However, without any ill-feeling, Kazuya cares about her like real sister. Also, even if he shows his feeling and wanting to protect Kanon as her brother immediately, it takes quite a while to show that to Kanon's father. (Emphasis mine)
From comment no. 65 on a Japanese blog,
一也の両親は離婚。ママリンは女手一つで一也を育てる。果音の母親は病死。パパリンはそれもあって娘を溺愛。兄妹二人がであったのが大体10年ほど前（新見さんより前ですね）。現在の両親は職場が同じで、それが縁で再婚。 [...] (Emphasis mine)
Rough translation:
Kazuya's parents divorced. His mother was taking care of him alone. Kanon's mother died. Her father also really cares for her. Kazuya met Kanon around 10 years ago (earlier than Niimi). Their current parents works at the same place and remarried by fate. [...] (Emphasis mine)
And last but not least, post no. 857 on 2channel,
一也の場合はママリンと父が離婚したんだよな。ロクでもない奴だったのか
Rough translation
On Kazuya's case, his mother and father divorced, eh. Worthless guy, wasn't he?
Unfortunately, I couldn't find where this is explained since I don't touch the series.
