Additional Family info

I had the exact same question when I was doing our application and I phoned the call centre a couple of time just to make sure I was answering this question right. They told me to state that my daughter even though she has not interacted or even met my husband was considered a step child. They also said that it didn't matter the age of the child even if I was 100 and my child was 70 its still a step child. Also my husband just received his medical request and an additional family form to fill out, I am not sure why he got either of them since he has not even had his interview yet.

Foremost, the additional family information form is informational only, and does not include anything that directly affects qualification for PR.

. . . so leaning toward disclosure is usually the better policy, and disclosure with explanation if you are not entirely comfortable with how you choose to answer the questions. In other words, there is absolutely nothing to lose by listing your partner's children as your step-children. But there is probably no penalty or problem, either, if you fail to list them as your step-children.

Personally, especially since you have a parental relationship with them, I would.

Notes/commentary re additional family info generally:
This really is simply for informational purposes, which, I believe, is primarily to assist CIC in positively identifying the applicant (which is actually, I think, the largest concern and focus of the examination process, which may seem odd to many but the use of false identities is probably the most prevalent and problematic issue in immigration fraud, so a lot of what the examiners do is geared to positively identify persons seeking permanent status in Canada), but of course it also provides a lot of contextual information against which the rest of the information provided can be weighed (allowing, though none of us know for sure, that the security check process may use this information to facilitate the background/security check). But it is only informational, not material to qualification.

Notes/commentary re step relation issue generally:
There are indeed many nuances to "step" relations, yours being just one among many, and since I too had a question (well, actually in my situation, several questions) about who had to be identified as a step relation I did quite a lot of research (just about two years ago) into this, with very little concrete results (and I do legal research for a living,, been doing so for several decades, so I have a pretty good idea about how to do it).

Who is considered to be in a "step" relation often depends on why the question is asked, which area of law the question arises in. For immigration purposes, I found nothing to clarify what they mean by "step" relations. It will also vary depending on the jurisdiction one is in (that is, what country, or state, or province). And there are many, many permutations of who might be a "step" relation given the tangled nature of relationships these days. Bottom line: except in the area of criminal law (since there are specific criminal offenses for which a step relation is material, and in the criminal law area things like this tend to be more precisely defined, but in that area the definition of a step relation is fairly narrow), there does not seem to be any precise definition for who is a step relation.

The obvious concern for some people arises from having a step-relation who has serious legal, especially criminal problems, thus raising the fear that mere association with them could cause more intensive scrutiny, and result in delays.