Yes but you don't find that out until the very end of the game. This is something I have trouble with. In my opinion this shouldn't have the "incest" tag. Tag it as though you're at the start of the game, unless they explicitly state a spoiler.
considering the time frame the game came out, i don't think it's that big of a deal. yes it is a spoiler, but you have to also consider that the tagger's state of mind would be: "a majority of people have played the game already".
However, it's giving information that it shouldn't know (and certainly the image doesn't provide any clue towards it). Danbooru has a pretty strict no spoiler policy, and this certainly qualifies as a spoiler (even if it was rather predictable).
The other option is tagging the image with "spoiler", but in my opinion it's kinda senseless to do that just because of a tag.
There is a statute of limitations on spoilers. And it's still incest whether most are aware of it or not. I would wager that the average person searching for Phoenix Wright material has already played the games and so is aware of the connection, even though this dojin was made before the game was released.
Dragon_Samurai said: hence my point on a majority of people who are interested in the game have played it already.
I guess I'm not the majority, then. I've been interested in these games sense they first came out on the DS ( the first Phoenix Wrights, I mean) and yet I'm just now finished Ace Attorney.