Well, you have to mention all these names and details of half brothers and sisters, however as the guide says, if these details were not mentioned, you and them will loose the right in the future of claiming points based on adaptability.
Since we don't know how the future looks like. I suggest you provide the details to the best of your knowledge and provide additional explanation letter. By doing so, you mitigate a risk; the risk that, hypothetically, some of your half brothers might be in Canada and you don't know, then CIC might read this as wrong discloure, negatively.
Cheers
I understand your point, but on the PIF form there is only a section for sister and brother (no section for
half-sister or
half-brother), do I simply make a note on the form that these persons are "half" brother/sister?