Parental Addressing of Kids

[PracTz]

I've seen the reruns and it bugs me that, at least in the early years, Terry and Rusty always addressed their stepmom Kathy as 'Kathy' while Linda never called her stepdad anything but 'Daddy'! The elder two were old enough to remember their biological mother Margaret before she died but why call Kathy by her first name when their stepsister called their father 'Daddy' from the get-go? Maybe she was too young to have remembered her actual father when he died but still it seems a bit much that Linda had her original surname changed to Williams when Kathy married Danny without either her or her mother expressing any regret over her actual late father's surname being discarded.However; it seems a bit unbalanced that the elder two were allowed to address Kathy by her first name -yet all three were expected to treat each other like they were biological siblings. If Terri and Rusty didn't want to forget Margaret, why couldn't they have called Kathy something like 'Marmee' to acknowledge her being their new guardian? Come to think of it,though, I can't think of any time Terry or Rusty actually mentioned Margaret after Danny's remarriage.
Feel free to comment and/or correct these assumptions!