How much is a step-parent required to pay child support when the biological parent is not paying child support?

Section 5 of the Federal Child Support Guidelines [SOR/97-175, as am.] (the "Guidelines") provides: 5. Where the spouse against whom a child support order is sought stands in the place of a parent for a child, the amount of a child support order is, in respect of that spouse, such amount as the court considers appropriate, having regard to these Guidelines and any other parent's legal duty to support the child. Whereas, in Chartier v. Chartier (1998), 1999 CanLII 707 (SCC), 43 R.F.L. (4th) 1 (S.C.C.) Mr. Justice Bastarache at paragraph 42 stated the following: ...The contribution to be paid by the biological parent should be assessed independently of the obligations of the step-parent. The obligation to support a child arises as soon as that child is determined to be a "child of the marriage". The obligations of parents for a child are all joint and several. The issue of contribution is one between all of the parents who have obligations towards the child, whether they are biological parents or step-parents; it should not affect the child. If a parent seeks contribution from another parent, he or she must, in the meantime, pay support for the child regardless of the obligations of the other parent...