Consensual sex between adult brother and sister for example isn't abuse. If it results in a child it is also unacceptably likely to result in birth defects because that's 50% DNA commonality. Consensual sex between parent and (adult obviously) child is more arguable because there's a significant power imbalance which would usually not be present for siblings, but it might not be abuse.

Cousin sex is just not a big deal, and especially beyond the 1st cousins with zero removal, ie the children of your parents' blood siblings. When it comes to stuff like "She's the daughter of my great-auntie's oldest boy" it's negligible. In some societies that wouldn't be tracked, everybody is a cousin and nobody is. Americans are weird about this. Rudy Giuliani for example married his second cousin. I don't even know the names of my second cousins. If I met one in a bar I'd have no idea. But in the US somehow that counts as strange.

When cousin having kids together becomes normalized, you get a lot more defects a generation later - when kids of cousins have kids with other kids of cousins in the same family.

It is not a non issue. The communities where marrying cousins is normal do have this issue and have significantly more severe disabilities.

"Cousin" is a vague claim. A parent is 50% similarity, a simple first cousin is typically 12.5% but may be higher if they're also related on the other side (e.g. Einstein married a woman whose parents were, respectively, a sibling of one parent and a cousin of the other, that's a lot of shared DNA). But second cousins may be only 2-3%.

So there's a huge gap between "Your mum and dad both have twins, and there was a double marriage, so, she's your first cousin twice over" and "She's your great-aunt's child's youngest" and yet you might get told both people are your "cousin" for lack of convenient terminology.

I think it's implied it's a lot of first cousin stuff and if you iterate this it starts building up in goofy ways if it is kept self contained enough.

Cousin sex is what gives you the Habsburgs. It's why Jewish folk are more likely to have certain genetic disorders.

It's harmless if it's occasional, but if it's repeated over and over again problems start building up.

I guess?

Label it whatever you want. It's still consanguinity and it causes a tremendous amount of disease and the largest offender by far is cultural acceptance if it.