I think it has something to do with adding relation after group npc generation in the wilds. I'm pretty sure I have never met(fuck) them before, and I have never released anyone or let them escape. It wasn't a single occurance either, there is at least 2 npc in my mansion who has given virgin birth to multiple children (in the wilds).
Thanks for the reply. For now I will just edit my save to remove the mother-children relation.