Nowhere in the tags ("Adult, Animation, Erotic, Hentai, NSFW, Ren'Py, Romance, Sandbox, Slice Of Life"), description, or screenshots does it mention the incestuous themes, mental instability, or grief issues. The only mention of the former is in the comments below - and your response implies that it's a translation error rather than a blood relation, comparing it to the Japanese habit of calling someone "onee-chan" (which is often used for friendships as well). Claiming, "you weren't tricked into downloading it" is provably, objectively incorrect - the content is not clearly labeled. The only thing resembling a warning is the mention of pop culture references at the end of this page's description.
That being said... I'll freely admit my overreaction was probably out of line; my apologies for the vitriol. And it's an easy fix with some content warnings in the description, visible before downloading - warn people about darker subjects like death, mental illness, and the implied dubcon/noncon in the screenshots (to say nothing of the aforementioned direct, blood-relation incest with the MC's half-sister). Blaming people for not hunting hard enough for said warnings, especially when they don't exist, isn't a good look.
For the record: everything I mentioned in my initial post came from the first few minutes of the game. I didn't "force myself through" anything. And the other games I've played with the aforementioned taboo themes mentioned them well in advance. You might want to consider doing the same, because beyond that initial shock, this game does look like it has a lot of potential.