B.C.
You and Dad need to be on the same page.
When she's working - she's an employee.
As an employee - she should have a job description of what her duties are.
If you get lip when you are asking her to do what's described as her job - she gets a disciplinary action like any other employee would.
Ultimately - she should be able to be fired like anyone else too.
Sometimes working for family is a bad idea.
She might NEED the experience of working for a real boss who won't give a darn if she's asked to do anything she doesn't like to do.
There's nothing like a boss from hell to make you appreciate just about every other job you've ever had.
Sounds like SD needs a transfer out of the family firm for awhile.