Whether the birth parent and/or child know about and/or know each other or not, to me, is not the point. I don't understand how they could require information about someone who is not your relative--in this case, this person you are listing is not your child, and is not related to you, legally. This is about additional family--and this person you are listing is not family. Having access to information from before you were adopted does not change the fact that the parents listed on a birth certificate are the legal parents, and everyone else in the world (including birth parents) are not. Just because you know your birth mother's name or have even met her doesn't make her your legal mother, suddenly.
In any case, I feel like this info requested is irrelevant, really. I'm pretty sure the reason they want to know about family members is that there is a chance those people could apply to immigrate to Canada in the future. I'd bet they like to know how many people they are really saying yes to, if they let you in. Because you might sponsor your family members in the future. And I totally get that. But guess what? Since you're not legally related in any way to this person they are asking you to list on this form, and thus they are not a family member of yours, there is no way you could ever sponsor them. Even if you meet sometime in the future, they will never be your legal family member. Not a brother, not a sister, not a parent or grandparent, and not a child of yours. Just legally, another random person in the world that you may or may not know personally. Sooo... Why do they need this information?