I just wanted to pop in and say that I never said the relationship was incest (although I guess I might have implied it), just that I personally didn't like it. Knowing Iori is adopted (although volume 1 never mentioned it so I had no idea) lifts my concerns somewhat, but given they've been raised as family it's still a bit 'ehhh' to me.
It's just a personal preference thing and the reason I didn't go hard on the 'it's incest, it must burn!' is because I do feel like these kinds of relationships can be written well and there are times I'm okay with them. Grand Blue just isn't one of them, but it wasn't even just because they're related (adopted or otherwise), but simply because I don't think the series was doing a good job at making me want to back the relationship.