I also think the "sweetfish river" might refer to a road, as well. If it's something that goes up and down, can be seen on a map, and ISN'T a literal river, what else could it be? It needs to have some kind of resonance, literal or metaphorical, with a "family tree" as well. Possibly due to a name associated with it or something near it, or something to do with its historical significance?
One of the things that interests me about this part of the riddle is "shore", which, from Eva's dialogue was clearly an important of solving the meaning. The reason I find it so interesting is because "kishi" is a word Ryukishi has played around with before, in one of Frederica Bernkastel's poems ("There is only one thing that I seek./ What I would obtain will either be the kishi (shore), or shiki (my death)."). There are lots of other resonances with Umineko in the Bernkastel poems, so I can't help but wonder if there is some kind of clue here.
The other interesting part of the riddle to me the "gouge and kill" sections. Unfortunately, I just don't know Japanese and so I can't grasp what's going on here as much as I wish
but I what I assume is going on here is that the anatomical body parts refer to either kanji character or PARTS of the characters which can perhaps be removed, turning them into different characters all together. I don't know ... but I feel there has to be some kind of weird Japanese pun, joke, riddle, or child's game which connects these parts of the body with kanji formations.