reposting as requested by Crinict
My sims have 3 children together and at some point between the last one being conceived and being born, "brothers" was added to their relationships list. When the child was born, the birthing parent got a milestone for having a child descended from grim (not true), and the child's relationship to the other parent is fixed as nephew and uncle.
The sims still consider themselves married, but cannot do romance interactions.
I have no mods installed, and every pack
While the other sim was pregnant, he did get and use the skill mastery perk "skillful sleep". I am wondering if turning into a ghost briefly has something to do with this, because as far as I know ghosts can't have children.