Ok, once you have info on a council member, how do you use it?
		
		
	 
As I remember, talk her/him about the laws, request, and there will be the possibility to mention your blackmail info.
	
		
	
	
		
		
			So, can NPCs have family members or not? For example, two NPCs being siblings? Because there is an achievement in the game that indicates that, but I've started the game, I introduce myself to every stranger NPC, and all of them only have non-family relations to each other
Also I've noticed there is a mother mentioned in the achievements? How do you unlock that? The MC only has a sister at the start
		
		
	 
They can have. Sometimes the RNG goddess smile at you when a new NPC spawn, so an NPC you know and meet will say, "hey, look here, this is my relative". Mostly I've seen aunts and nieces.
Or if you bribe a council member, one method is to offer a job his/her "idiot kid". It could be a daughter, if he/she old enough to have an adult kid, or if too young, a sibling.
As Humblebien said, if you employ young strippers, their mothers could be spawn to harrass you.
Lastly, your mother spawn cca. 15 month ingame. Or later, because RNG.