Does a step-parent who has a relatively brief marriage to someone who already has children have an obligation to support the children to the end of their post-secondary education or longer if they have serious health issues?

I must say that I struggle with the principle that a person who has a relatively brief marriage to someone who already has children and, acts as what Topolniski J. characterized as a “good step-parent” to the spouse’s child or children (in Shirley v. Shirley, 2007 ABCA 281 at para. 3), thereby assumes an obligation to support the children to the end of their post-secondary education or longer if they have serious health issues. That obligation apparently co-exists between or among the children’s biological parent, and presumably with anyone else who might later stand in loco parentis to the children.