Okay. The son has become a child and a new daughter has been born... and I think this little experiment is over.
It's a bit Messy so let's put it like this:
Daddy Muthkat immigrated with his two daughters. Their mother is unknown (ungenned). So he has no relation to her. Instead he married Tirist and had two children with her. In his eyes there is a wife and 4 children. Happy family of 6. Easy as that.
Mommy Tirist fell in love with Muthkat and, after isolation, became friends with his two daughters. She figures they are his and doesn't adopt any mothership role. She only considers the ones that passed through her body as her children. Her relationscreen even states "only son" and "only daughter"
The children are: Avuz & Logem (immigrants) and Tekkud (male) & Meng. Avuz & Logem don't take any notice of eachother or their real (non-existant) mother and move around like normal dwarves. They don't take any notice of neither Tekkud nor Meng either.
Tekkuk & Meng are happy in, what they consider to be, their family of four. They don't acknowledge the immigrant girls .. and don't seem to build any new relation with them.
Since the fortress-borne ones hang around Mommy Tirist all the time and the outsider girls prefer to stay in their own rooms .. the kids don't interact that much.
I put the girls' beds into the master bedroom, to make the children speak to eachother and spark up a friendship. So far there has not been any change in their relations, despite them hanging out all day. I don't think there'll ever be dwarven incest.. because it seems the girls are indeed siblings to the fortress-borne and cannot make new connections, it's just that they're ... bugged. and thus they don't understand family values.
edit: Oh god. I read my post and it sounds like I consider incest to be normal family values.
Aspgren retches.