I get that, but, you're playing a pron game. An early access one. One that let's you, decide your text relation to the character. Maybe a toggle should be added, but you're having a strange struggle of your own.
I can't help but think, they were written as a roomate first, a family member second, and the dev hasn't gotten around to properly drawing the lines between, datable family, and incest.
I think it was intended to open it up to people who would date a cousin. Imo, if it's your blood related uncle's, brother in law's daughter, a distant not-blood related cousin, is acceptable. Then there are wierdos who would date what is basically their sister from their uncle.
And even then, your sister, could be a sister in law. Which, is a really, really taboo relationship, but as far as I'm aware, completely legal.
Again, there ought to be a toggle on/off if you think of your roomate as bloodrelated or do not enjoy the taboos, but dating a cousin, or sister in law isn't necessarily illegal. So...