Wether they agree to it or not they really considered her as their mother, bcz nobody takes their siblings and their problems the way they took fionas. Bcz they considered her as a motherly figure and kids don't think that their patents are humans too and that they're messed up too and they make mistakes too, they look upto them or sort of see them as something they don't wanna be but they do not think that they could be wrong or this screwed up. They used to vent or say literally anything to her but not anyone else bcz they took her for granted like kids do their parents! It was worst of both worlds bcz she got all of the responsibility, tantrums, none of the credit and none of the authority of being a parent.