Since some factions get their new generals through adoption, could children and siblings that don't show up on the family tree be represented by traits (or non transferable ancillaries: it always seemed strange to me that a drunken uncle could be passed on to someone else in vanilla)?
Suggestions would be things like protective older brothers giving a boost to security, lots of adult daughters giving influence through marriage, jealous brothers plotting lowering personal security, clever relatives aiding management, useless or sickly children reducing happiness because the succession isn't secure etc.
I think this sort of thing would help when imagining that the family tree only shows key characters and your generals aren't actually all infertile.