A real example: (names changed)
Zoe Smith is the child of John Smith and Emily Williams but Emily is now living with Fred Wilson and had a child with him called Joe. Joe Wilson is the brother of Zoe Smith and he has Emily and John down as parents whereas Joe has Emily and Fred.
Now Fred and Emily have Chloe living with them, Chloe is Fred's daughter with his former partner, Susan Jones and retained her mother's family name. However, Emily has parental responsibility for Chloe Jones and so she should be down on SIMS in that case. So should Susan as although Chloe is not living with her, she still is her mother.
So. Zoe Smith; Joe WIlson and Chloe Jones are all siblings but different people living at different addresses need to see their records.
If Emily comes into the school office and says that she has a court order that prevents John seeing Zoe how does that get recorded in SIMS? And does that prevent the school from sending information about Zoe's progress?
So relationships are more complex than in my day so it easy to see that it can get screwed up. And in education software there are fixed release dates set around events in the school year so the pressure is on.
However, there is this thing we have all heard about called testing...