Siblings and parent conversations are auto generated by the game, there is no need to implement those.
That being said, sibling/parent conversations also override whatever conversation type both units have, so for example, say Male Corrin has a fast romantic option with Caeldori, but if you marry him to Subaki, that fast romantic conversation is now a non romantic parent/child conversation.
As for parent/child, it's just as easy as writing down the character ID of the desired parent in the child unit's character block where the "parent ID' byte is located.
Ah, that's rather interesting. So assigning parents through custom DLC is possible? I imagine it wouldn't for say, giving Lissa the ability to parent Odin, correct? But we could have Lissa parent Owain by specifying that in Owain's data?