haha they adopted, it's all laid out in the letter at the beginning of Book 2. Their biologies are too different to procreate together, but raising kids isn't about whose genetics are whose, and there were a lot of orphans in the first few years of the City.

Haha, I hadn’t even thought about their biological differences. I have to admit, the thought of their raising a dozen kids of whatever origin is hilarious.

Just the two kids is about what they can handle, but so far it is pretty funny. Drawing little Dai play with his shape has been great fun :) And Talia is a surprisingly fierce big sister :D