It would be far from the weirdest fetish on this board, but no, I just get weirdly obsessed about definition and still had the LUCA (Last Universal Common Ancestor) and humanity's evolutionary bottlenecks bouncing around in my head. We are all related, and humanity specifically isn't just hairless apes, we're fat, hairless, inbred apes. (The minimum percentage of body fat a human body needs just to function without issues is above the percentage of body fat most other apes can gain naturally.)
Hence, again: Shades of gray. And since I tend to look stuff up before I post because I get anxious about posting false facts and tend to fall down rabbit holes…
Anyway, the actual definition of incest is different depending on culture. Heck, technically, the Habsburg line never committed incest, "only" inbreeding. Incest requires being "too closely related to marry", and you can bet that the Habsburgs were always formally married. That was kind of the point.
And considering that the question is "How far can TiTS go without Patreon nuking it" (it actually contained incest content before Patreon banned that stuff), it's kind of relevant to know where they draw the line, and the most likely line is "No parents/siblings/children", because that's the only line that's actually pretty universal.