For me is Cinderella and Stepmom .. two different roles played by the Stepmother.
But most effective role was of course both the mother (Susan Sarandon) and Stepmother (Julia Roberts) in Stepmom .. Both mothers were competing on who is liked better by the children. And yet Julia Roberts as the Stepmom went an extra mile to show love and compassion to the sickly mother.
But we will not know past the movie because Cinderella actually shows the Stepmother showering more affection towards her own flesh and blood compared to the stepdaughter. So if Julia Roberts had a child will she be a good mother ..
Probably she would because from Julias character she has proved that she is able to love the ex-wife.
Conclusion .. love of a mother and step mother towards the children in very individual .. Who they are makes the difference, if they have care and love for a child they will never differentiate between 'Own' or 'borrowed' ,,,