When a person enters a relationship with children, and they take the place of the missing parent, then yes CS can be requested. It's called "Loco Parentis".
She would have to show that you acted as a parent in all senses of the word.
IE taking the child to medical/dental appointments, signing permission forms, attending school meetings, discipline, etc.
I would ask that the particulars of the Bio-parent be entered into account to determine if he "is" capable of paying support. Maybe she just walked away from the relationship and never asked him for anything, maybe he disappeared and she was unable to seek support? You'd have to get your hands on some info. The reason being is; if the courts deem that you had acted in place of a parent they could order full CS relative to your income and the CS tables. If the BIO is found to be able to contribute, then the total table amount for a single child less his share would be your CS responsibility. I know it doesn't seem fair that he gets off and you foot the bill. But that's the way family law works/. There is no law that states that 6the parent MUST seek support for the Bio parent, but you can request that his employment and financial records be ordered so that it can be determined what he is responsible for. If you're lucky he gets the order for CS and you don't.
I'd be very curious to know why she never sought support from him?
Maybe there is a way you can use her reason for not seeking support from the BIO to benefit your case???
Here's a couple of links to significant cases that will explain the "Loco Parentis"
http://www.canlii.org/en/ab/abqb/doc/2001/2001abqb1079/2001abqb1079.html
http://www.canlii.org/en/bc/bcsc/doc/2001/2001bcsc1198/2001bcsc1198.html
Here is a link where the bio did not pay support until the mother sought support from the step father. Step father was order to pay support, but so too was the bio father.
Worth reading to educate yourself on how the courts tend to rule.
http://www.canlii.org/en/bc/bcsc/doc/2006/2006bcsc1160/2006bcsc1160.html