According to the rules posted, as long as you were not a spouse or common-law partner at time she landed, and for this app the undeclared child is a dependent of the principle applicant (you) and not being directly sponsored themselves, then it may be possible.
In practice I have not seen this situation very often, in fact just the 1 case someone mentioned that the visa officer used this rule to allow the sponsorship. But it seems based on the rule your case should be allowed. I don't know if there will be any complications, or the misrepresentation on your step-mother/spouses's app (not declaring the child) will be an issue.