is it ok for a brother and a sister to sleep in the same room as their father and stepmom and have their half sister out on the couch

Unfortunately, I cannot answer this question.  It depends.  It depends upon the age of the children.  Brother and sisters should normally not sleep in the same room once they are old enough (no real age, but I would say around 5 or so).  If there are sufficient sleeping facilities, they also should not sleep with adults.  This is all dependant upon ages, income of parents, size and layout of the home and other factors.  The best interests of the children are the determining factors.